If you need directions, stop in at a pizza restaurant that delivers. The delivery people really know their local area.
If you're traveling with several people, take an over-the-door shoe holder with clear pockets. Hang it up at your destination and use it to hold room keys, cameras and phones, sunscreen, hats/gloves, toys, snacks, etc. Makes them easy to find and keeps the room neater.
I bring a number (varies depending on the length of travel) of $2 bills. Dollars aren't worth very much in many places, but $2 bills are a novelty. Two singles aren't much of a tip anywhere, but a $2 bill is unusual enough that the recipient won't mind.
If you are traveling alone and/or on a budget, eat your midday meal at a fancy restaurant and have your evening meal somewhere simpler. Better yet, go to a local grocery store and buy things for a light dinner. You learn a lot about a culture from the food stores.
Pack your carry-on bag with duplicate cosmetics, foreign electrical plugs, a battery-operated shaver, a worldwide current hair dryer and curling iron, basic medical supplies and pharmaceuticals in separate zipped bags. I keep all liquids in the required quart bag all the time. Then I simply add whatever new I must have for the upcoming trip such as mosquito repellent. When I get home, I restock what I have used, put washable items in the dishwasher, then put everything back in the bag. No repacking for every trip.
When traveling internationally, be sure to check out the CDC website (wwwnc.cdc.gov/travel/destinations/list.htm) in advance to ensure that you have the most up-to-date vaccinations.
I always take a small roll of duct tape. It can often be found on the hardware shelf of your local grocery store. The tape works wonders for a quick fix on a torn hem or for taping together a piece of luggage with a broken zipper.
If you are driving in Europe, buy a plastic-covered map. Paper maps become torn and tattered from constant opening and closing.
Check with your airline ahead of time to see whether there's a weight limit for carry-ons. I bought a Rick Steves convertible carry-on for a trip to Italy and assumed it was the right size for Lufthansa. As long as I didn't stuff it, I thought it would be fine. But a coworker who travels regularly to Europe suggested there might also be a weight limit. I converted the carry-on kilogram weight limit on Lufthansa's website to pounds and realized the limit was 17 pounds. I was considerably over that and was very happy to have found out ahead of time, rather than at the airport.
Readers, keep those tips coming. Send your hard-won nuggets of wisdom to firstname.lastname@example.org. Include your name and city of residence.