The big cities - San Diego and Los Angeles - abound with gay nightlife, not too mention countless opportunities for shopping, culture, and great dining. Head to Hillcrest and North Park in San Diego and West Hollywood in Los Angeles for the best in socializing. In between, the laid-back gay-popular coastal town of Laguna Beach, in Orange County, is well worth a visit, as is underrated Long Beach, which has some cool gay bars, fun coffeehouses, and a friendly, easy-going vibe. Check out coastal Venice Beach and Santa Monica once you make it up to Los Angeles, and continue north for great beaches, hiking, and wineries in Santa Barbara and - if time allows - clear up the central coast through San Luis Obispo, Monterey, and Santa Cruz, before reaching the San Francisco Bay Area. You really can't go wrong choosing any stretch of coastal California for a spring break road trip - just bear in mind that the weather is sunniest and most pleasant farther south.
Road-tripping as a spring break vacation isn't for everybody. It's not necessarily any more or less expensive than spending a week in one place, but be sure to factor in the cost of gas, rental cars (as applicable), and the fact that it's sometimes pricier to book a series of hotel rooms in different cities than a one week-long hotel stay or condo rental in a beach resort. On the other hand, if you're traveling as a group of three to five friends, you can split the costs of and road-trip quite economically.
Factoring in the weather in March and early April and the opportunity for great scenery, a welcoming gay vibe, and fun nightlife, Southern California is the best gay spring break road-trip region of the country. However, as an alternate, you might consider a trip in the Southwest through the Four Corners region (some combination of Sedona, Grand Canyon, Monument Valley and Canyon de Chelly, Bryce and Zion, Arches and Canyonlands, Durango and Mesa Verde, Taos, Santa Fe, and Albuquerque. The Four Corners is spectacular, though it's almost entirely absent of nightlife - think of it as the ideal nature adventure.
The entire southern half of the country is worth considering for a road trip during the spring break season - the Carolinas to Georgia coast would work (think Wilmington, Myrtle Beach, Charleston, and Savannah), as would the Gulf Coast (Pensacola to New Orleans). It really just depends on your interests.