My 18-year-old brother is going on his first semi-solo backpacking trip to the UK in a couple of weeks. He’s planning to be gone for two months, and he’s traveling on a budget. So, Isaiah, here is some packing wisdom from your oldest sister, who happens to have spent about two years of the last 11 living out of suitcases schlepped through the flights, subways, trains and busses of five continents.
When I was 19, I started out with an all-purpose red backpack, large enough to hold a thick poncho and an assortment of clothes, but small enough to fit into the overhead bins of most airlines. However, I realized after awhile that it gave me back pain and marked me as, well, a backpacker, and at least for that first trip to China at 19, what I had brought with me made me feel really out of place and ugly. I didn’t want to wear hiking boots all the time, for example. So I moved on to a variety of shoes and rolling suitcases, reasoning that I would mostly be hauling my stuff on flat surfaces, and that any hiking I would be doing would be on day trips. Rolling suitcases are great for a number of reasons, but they do have one drawback: the wheels can break, especially on cobblestones, and it’s not fun to run for your train clasping your broken suitcase in both arms.
My last few trips, I packed very carefully in rolling suitcases that fit Ryanair’s cabin requirements so that I could purchase cheap transportation. I am a smaller woman, so my clothes don’t take up too much space, but I also packed a largish laptop and a largish camera in my cabin bag, so I can say with a good degree of assurance that it is possible for most people to squeeze everything into this, if they’re willing to do the wash frequently.
But it is important to take care of yourself, especially your feet. You, Isaiah, may need to shop for a good pair of shoes. I recommend leather for class, for breathability and water resistance, as well as good soles for comfort. Nothing too heavy. If you find the right pair of shoes, you only need one. Break them in before you leave. Almost nothing is worse than being stuck with one pair of blister-inducing shoes when you’re supposed to be walking everywhere. Just in case, bring band-aids, and a pair of sandals or flip-flops for when you’re relaxing. Or for when you need to take a shower in the sub-par hostel you’re staying in without contracting foot and mouth disease.
Bring good socks. I recommend wool socks, athletic-grade, at least if you’re going to the UK in the fall. They stave off odor, are warm, and help keep your feet dry and unchafed.
Bring clothes that travel well and that you don’t mind wearing on repeat. Brands like Patagonia and North Face can actually be quite good for travel, because they have been built to be smashed into packs, worn for days on end, and still look good. They’re usually compact, too.
One of my all-time favorite pieces of clothing for extended trips is a dark blue Patagonia dress. The one thing I didn’t like about it was that it tied in the back in a knot, which made it slightly uncomfortable for sitting around in. So I altered it by hand. And then, at the Storm Cellar, I found this silk crepe blouse of gossamer sheerness that made it look dressy. Without the blouse, I wore the dress hiking, boating, wading, sightseeing, and to dinner at maybe the best restaurant in Rome. With the blouse, the dress was perfect for the Catholic wedding and the Parisian conference I attended in the same trip. And for sitting around with my family in Iowa.
Bring layers. Bring a good coat; either something that’s water-resistant with a hood, or a down coat, if it’s less likely to rain on you and you know you’re going to want to put it in a stuff sack and use it as a pillow. Bring a small towel; it could even be a ShamWow. Bring hand sanitizer. Bring hand lotion and chapstick and toothpaste in travel-sized amounts. Bring a small canister of baby powder or dry shampoo. Bring Ibuprofen, Benadryl, and vitamins. Don’t bring expensive jewelry — or your flashy pocket watch. Do bring a money belt and a coin purse, so you don’t ever have to go digging through your money belt in public. Do not leave your valuables alone in the hostel, unless they’re locked up in a decent locker. Do bring a day pack of some kind — my most recent one is a black neoprene bag with a zipper, into which I could fit water, my camera, granola bars, a jacket and so on, but which would also collapse into a small enough mass that it could go in my suitcase too. Bring a small internet device, such as an iPhone, if you have one. Disable your cell data and just use wifi, and you won’t get any international charges.
Traveling with less luggage is cheaper. You won’t have to pay checked bag fees, you won’t be tempted to take a taxi rather than the subway, you won’t have to rent huge storage lockers, and possibly most importantly, you’re less likely to get robbed. Thieves tend to target people who are juggling a bunch of stuff and who otherwise look like tourists. Quite simply, try not to look like that.
You may have to invest in good travel gear, but in my humble opinion, it’s worth it. If you’re planning on spending $500 on a rail pass so you can bring a huge pack, and can instead spend $200 on short-distance individual bus and rail tickets, and $100 on Ryanair flights for longer distances, your travel allowance alone might pay for it. I’m not guaranteeing that; check out where you’d like to go and what the best investment is for you. Do your research before you go. Often, you’ll want to buy the tickets in advance.
And use CouchSurfing. Be polite wherever you go, and listen more than you talk. Bring your hosts a small gift, if possible. You’ll learn more that way.
Botkin Creative 
I disagree on the leather shoes. I’ve gone trekking in Nepal wearing super-lightweight Nike Free Run shoes (all black) and also worn them to formal dinners with jeans and a nice shirt. They weight nothing and scrunch up well if you have to pack them. Leather shoes smell, are heavy, and don’t hold up in bad weather. Forget leather. As for socks, get a couple pair of SmartWool socks–they’re expensive but they will keep your feet dry and comfortable and will last a very long time.
I think it depends on the shoes. I’ve worn leather shoes that were really uncomfortable, but also, some of my favorite traveling shoes have been leather. You can waterproof lightweight leather boots pretty easily, although I would not recommend leather boots for hotter climates. But your Nike Free Run shoes sound good… although I don’t think I could have worn them with a dress.
And I completely agree about SmartWool. Love that stuff.
I should add on further reflection that possibly my favorite leather shoes were woman-specific. For instance, a Mary Jane with a tennis-shoe sole. Or tall, flat-bottom boots of supple leather.
Make sure you take traveling underwear. Low odor, dries super fast, and packs light. Wool socks can be worn multiple days in a row without odor but dry very slowly when you have to wash them (or get water over the top of your boots). Check Sierra Trading Post (sign up for deal e-fliers) for bargains. I regularly save 40-80% on gear, clothing, and the like. When it comes to layers, I tend to go wool short – fleece – shell (ie wind shirt or rain jacket), but that’s for backpacking in the mountains. Still, if you think in terms of loft/insulation and shell/barrier over top that should help you break it down.
I could come up with a list a mile long of things I did while traveling abroad. Aside from a list that included packing a bomb in your suitcase, it is probably the best resource you’d have for Things Not to Do.