From the West Coast, your main options outside the continental U.S. are Hawaii, Mexico, and Central America. Alaska (often codeshared with Delta) and Hawaiian both have nonstop service from Seattle to Honolulu. You can reach the outer islands and most of Mexico and Central America with a single connection in the West.
Prices will vary sharply with the season, however, and not just in the sense that warm weather destinations cost more in winter. For example, your time frame of late March is rather unfortunate. Getting away from the cold to relax is what millions of college students are also doing in March, and as Easter falls on March 31, 2013, many families are as well. You can often fly SEA-HNL mid-week for under $500, but not in that time frame. Aim for the week after President's Day or April to hit the shoulders.
Similarly, you can get to Costa Rica for a bit more than the cost of going to Hawaii— in October, during the rainy season. For breezy January, expect that price to go up 30% or more.
Since the demise of Travelocity Dream Maps, my usual tool for such planning has been the FareCompare.com Getaway Map, which plots out destinations and prices on a map. Google Flights has a similar, more primitive functionality. Bing Travel, like FareCompare, has tools for tracking trends in fares.
As mentioned in a previous answer, Kayak has a deal finder— it does search for packages deals, just select "Vacations" under "Deal Types"— as do Hipmunk, Travelocity, Expedia, and so on.