Having gone a few places for various amounts of time I have a short list of things to pack:
Sunglasses – because you will have to buy them if you really need them if you don’t bring them.
Sunscreen – no matter what the weather, a sunburn hurts and will ruin your trip.
Tylenol – or whatever you take for soreness, headache, toothache, cramps, etc.
Binoculars – I always forget these and am always sorry. Even a small cheap pair would do.
Very good walking shoes.
Comfy clothing – do not pack items that are fancy, that you’ve never worn or are not practical. If you think you’ll need something nice, bring a dress or slacks and nice top. The problem I always have with this is shoes – now I have to pack dress shoes – and in the end, don’t wear any of it. Think of items like coats and computers on the bus, plane and anywhere you will have to carry them around for any period of time. Lugging crap around sucks. I wish I would have brought stuff I wear at home all the time – the sleep shirt or jammy pants, the worn jeans, warm slippers and old sweatshirt.
Most times we try to travel light – meaning carry-on only so we never have to go to the baggage claim or lug crap around (see above). This means one each: walking shoes, jeans, pants (eg. dress or khakis), sweater or sweatshirt. Two short sleeve t-shirts. Three pair undies and socks. Medicine, deodorant, toothbrush and paste, contacts/glasses. Hotels and the like usually have soap, shampoo and lotion. Optional: long sleeve t-shirt, a couple coffee or tea bags, camera, small collapsible umbrella, pen or pencil .
Now – pack and then take out 1/2 the stuff you packed. Remember some of the clothes you are taking, you are wearing. Things that seem like a good idea sometimes aren’t. Ask yourself if you could do without it in a pinch.
Disclaimer: these tips may not be complete or fit your situation. They are not for specialty trips such as business, backpacking or long-term travel. Try to think about things that may wriggle open and spill on your clothes.