Sandtrap, a few additional tips (those above are excellent), been travelling international almost 3 decades now.
Footwear, key is to find sturdy, durable shoes, leather only if possible, and wear with two pair of socks, a light inner liner then a compatable merino wool hiking pair. REI is a great source of footwear for world travelers.
Wallet pouch: I spent plenty of $ on special travel pants, multiple zippers, etc. My best option has been to use an internal wallet pouch that connects through belt loops and flips inside. You can get these at any luggage/travel store in a mall. It's cheap and very effective. I use mine all the time and know my wallet is safe. I have a second I used for my iPhone.
Belts: speaking of which, you may want to get a sturdy belt with in internal zipper. I have a nice black and brown pair and keep abt $100 plus copy of my passport in there. Also have a "throw away" wallet I keep in an outside pocket with just a small amount of cash. I was pick pocketed in the Madrid metro station a ways back and glad they got the throw away and not the main wallet.
Pants: My most comfortable pants are the REI brand nylon hiking pants. They are a bit casual for dinner and such but great for walking around. They are light, very comfortable, zipper bottom legs so you can make them shorts, and they dry quickly.
Water: be sure to have a thermos or other water collection to keep while you are travelling around - you never know when you might pass a store to buy water. I wouldn't drink public water from a fountain, etc. outside the US or Canada.
Hat: something light and easy to use to keep the sun off your eyes and head. But don't look like an American tourist
Small backpack: I also travel internationally as carry-on only. I have my trusty old Travel-Pro I bough in 1991 (stapled, duct taped, etc.) but it works and fits in the overhead bin. I have a second travel backpack that fits under the seat. That's my daypack when in town, when travelling, I keep a change of clothes and basic necessities if I should ever get separated from my Travel-Pro (actually happened coming out of New Zealand to Hawaii and of course it didn't make the connecting flight, glad we had enough to tie us over until the next day). For the backpack, I have found a Targa at Best Buy or someplace that fit well and had two outside places to keep water.
That's all I can think of off the top of my head. I fly mostly United (our preferred carrier at work) and find that if I get one of the last rows, actually the very last row, I get in Zone 3 and if I get in line early I never have a problem getting an overhead bin. I have noticed lately that zone 4 and 5 for United have been running out of space which would require a gate tag. I like to get out of the airport as soon as possible.
Happy travels.