A guide book, travellers checks as opposed to too much cash, change local currency into small notes so you don't announce to the world how much money you have when you pay for something, travel light, keep clothes to a minimum and get some comfortable shoes.
Always best to do a little research and plan your trip and what you are going to do. Don't feel that you have to stick to the plan, as that is half the fun, but its better to know where the best places to go are before you travel. Nothing worse than getting home to find that you have missed something exciting.
Do a little financial planning as you don't want to run out of money halfway through the holiday, and depending on where you go, always drink bottled water. Avoid ice in your drinks, its better to drink warm coke than to spend a day hunched over a toilet!!