This is a guide based on my trips to Ireland, London, Japan, and Germany. I won't talk about the specific trips here but will more specifically talk about inventory / packing. I'll write some separate post about other aspects of traveling. The main theme of this guide is reducing mass, volume, and redundancy. This guide assumes you'll be staying at a mix of hotels and hostels.
The first thing you'll want for your trip is a nice backpack. Don't get a duffel bag or anything you need to dedicate a hand to; I did this when I visited Japan and I was constantly having to pick it up and set it down and hoist it on my shoulders. Strive to only bring a single bag. While it's true that you can have a personal item and a stow away item on a plane, you probably just don't need that much stuff and having two is such a hassle. Personally I use this Osprey bag, originally purchased for camping, but it's great for traveling. It has a padded laptop compartment and has external water bottle pockets.
You'll also want to have a Dopp bag which is a smaller bag that goes into your main bag and contains stuff that you'll need to use in the bathroom, such as shaving or toothbrush or whatever. When it comes time to shower you don't want to bring your massive bag into a cramped European bathroom. Bring a small combination shampoo / body wash if you plan on staying at any hostels as they don't always provide.
You should always carry a water bottle with you. When traveling you're going to be visiting unfamiliar places with unfamiliar temperatures and doing unfamiliar amounts of physical exertion. Each of these unknowns brings potential dangers to your hydration levels. For this reason always bring a bottle and keep it as full as possible (though you'll need to dump it out at airport security). I'm a big fan of Nalgene water bottles, 50 oz.
Buy a power adapter for the countries you're visiting. Here's a resource about the different plug types so be sure to get the adapters ahead of time. If you buy them on Amazon they're much cheaper (and higher quality!) than buying them in various shops while traveling. I bought one in Paris which broke within seconds of use.
Also make sure you have a good credit card which doesn't give you foreign transaction fees. I use the Chase Sapphire myself and like it, though most of my friends have already upgraded to the Chase Sapphire Reserve. You'll also want to bring a debit card. The debit card will allow you to withdraw cash at ATM machines. Some people call their bank ahead of time to tell them they're traveling but I've not had to do so with Chase.
Towels aren't always free at Hostels, so you may want to bring your own towel. That said they have always provided a towel for me for about 1 Euro so it's not really worth it to bring a towel if you don't mind spending a few extra bucks. I do have a quick drying towel which I use for camping.
Bring a small lock. Many hostels offer a locker but you need to bring your own lock. I also like having a mechanical pencil and a small book of paper to keep notes with. Whipping out the laptop or phone and using electricity each time you have an idea isn't that efficient, it's better to take small notes now then enter them all in later, hopefully while plugged in.
Bring earplugs so that when the selfish guy in the hostel bed next to you with sleep apnea who didn't pay for a private room starts gasping all night you don't have to stab yourself in the ears. If you find that those small foam inner-ear ones work for you then that is great. Unfortunately I was cursed with being a light sleeper and need something more hardcore, I use heavy duty hearing protection used for shooting rifles with.
Bring one pair of pants (e.g. the ever-malleable blue jeans), one pair of shorts, and a generic looking pair of sneakers which work with either. Also, bring Ankle socks, full length socks will look goofy with your shorts. Of course, as you're reading this you can probably tell I'm not the most fashionable, so depending on your goals for the trip (e.g. fall in love) you might want to make your own clothing judgements.
Bring a hoodie, they should save you during a hot summer night or a cool autumn day. If you're traveling in the winter you'll probably need a coat though. Personally I like to run Weather Underground and look up the locations I'll be visiting and the dates I'll be there. It's also useful to check something like Wikipedia to get historical information for longer trips. If you bring a hoodie or coat you should wear them while traveling, they might not fit in your bag. You might get made fun of but a hoodie can be easily tied around your waste while on the move.
Bring seven pairs of socks (preferably all the same type in case one or two goes missing), seven shirts, and seven pairs of underwear. This will allow you to travel for a week between having to visit the laundromat. Also bring a mesh laundry bag with you, something that can segregate your dirty laundry from your clean laundry, one with almost no mass that can be rolled up and occupy minimal space. For shirts I'm a fan of plain colored T-Shirts without wording; no words means people can't guess your nationality, and much like with programming, information hiding is always a good thing.
Bring a dumb watch. Knowing the time is always useful, especially when your phone is dead. What isn't useful is a dead apple watch hanging lifelessly from your wrist. Each time you arrive at a new train station or airport you'll want to check the local clocks and reset the time if you've switched between timezones. I have this Timex watch which is super cheap and has the added benefit of having an alarm. The battery will last for years. It does make you look like a crazy neckbeard, though.
Bring a hat, you'll want it to block out the sun. Using sunscreen is great too but it's easy to forget to apply. Always putting on a hat (specifically a baseball hat with a bill) you can rotate it to block out the sun. Anytime you're in the sun for more than a few minutes you should align it to the sun. It also collapses easily into your bag. Much like with shirts I'm a fan of plain hats without logos or text.
It's important to get a lightweight laptop with good battery life. I use a Dell XPS 13 laptop at home, but I wanted a disposable laptop, so I also have a secondhand MacBook Air “burner” to carry around as well. Both would do a good job and there are probably others of comparable quality. Don't bring an external mouse or keyboard, they're totally redundant with the “good enough” trackpad and keyboard in your laptop (also you'll get funny looks if you use them in public). However if you plan on working a lot and require ergonomics, get a split keyboard as it'll be easier to pack.
Bring a pair of earbuds on your trip, instead of the more acoustically accurate $500 pair of studio headphones you use at work. A pair of headphones weigh a lot and take up a lot of room while a pair of earbuds can be rolled into a ball and shoved into your bag. At the end of the day you simply need them for private audio listening, and perhaps tele-conferencing (get a pair with a mic). The apple earpods are my favorite, they never fall out of my ears, but YMMV.
Never forget ABC: Always Be Charging. This is where the trip becomes a resource management minigame. Ideally you only have a single wall adapter, additional ones add weight. In my case I need to charge a MacBook Air, a USB-C Android phone, and a USB-B battery. Ideally you could have entirely USB-C devices and fewer cables, like a MacBook or new XPS13 laptop, unfortunately I can't find a USB-C battery. One common pattern is that I walk around a city and occasionally charge my phone with the battery, try to go to bed with 100% phone charge, and keep the battery charging on a desk while I sleep with the phone in a pocket. This is good as it minimizes the amount of valuables left unattended, especially if in a hostel. I also found that in Europe it's rare to find power ports as compared to the US.
Bring a high capacity external battery! These things will save your life. I have this 10,000 MwH battery by Anker. This battery can quickly charge my Android and has enough juice to charge the entire phone at least twice. It has a button which will tell you how full it is. It charges pretty quickly as well, normally I'll get to about 75% by the end of the night and it can charge in an hour. When I'm at a hotel I'll plug in the laptop, and charge the battery and phone from the laptop. This isn't as efficient but by morning everything should be at 100%.
Protip: Whenever you don't need to make use of your network connection, put your phone in Airplane mode. This will massively save on battery life. Try to keep the backlight low as well. If using an Android, enable the Battery Saver feature (iPhones don't have such a feature but they use less power and don't need it). And finally I'd recommend against walking around listening to music, it's both a drain on battery and hides the sounds of the cool new world you're visiting.
Americans: You should absolutely switch to a carrier which uses GSM, like T-Mobile. The support for this carrier has been great in the countries I've visited. I had a Sprint iPhone for my trips to Ireland and Canada, and as soon as I crossed the border it became a paperweight.