If you are referring to days trips and that sort of thing, it will be much cheaper if you book once you get there. In Vietnam in particular it’s amazing how cheap tours can be if you book them through your hotel. I once did a 6-hour bus tour to Mỹ Sơn, which is near Hoi An, and I think I paid about US$6. It even included a boat ride and a basic lunch. That same day tour online would probably cost US$30 because the booking company takes a share and there is very little competition.
Traveling during the last ten days of December means crowds and exorbitant prices throughout much of the world, but the first two-thirds of the month—before holiday airfares and hotel rates go into effect—can be a serene, value-laden, and just plain lovely time to vacation. Europe is all decked out for the holidays—with locals generally friendly and in high spirits; in the Southern Hemisphere, spring is in bloom; and in the Caribbean, Hawaii, and Mexico, hurricane season is over, the weather is gorgeous, and there are free upgrades galore.