First, think European vacation. Right now British Airways and Lufthansa are running last-minute savings to Europe through the end of March. After that, the prices go up. Also, travel midweek for the cheapest flights.
In the U.S., if you aren't as focused on your tan line as you are on the bottom line of your budget, look for great deals in New York, Chicago, San Francisco, Los Angeles or Washington, D.C. Use tools like Fly.com's Fare Calendars to see when the best days are to fly.
If you are a ski bunny, the later in the season you wait the more you save. Check out resorts from right here in the Poconos to Tahoe, Vermont and Vail for late season deals.
Also, think wine country. It's off season in Napa and Sonoma, but that doesn't mean it isn't beautiful! You can save half on bed and breakfasts, hotels and other places to stay!
Also, apparently the latest Carnival debacle has not deterred families from cruising. There are great bargains to be had for remaining cabins on cruise lines.