San Diego is well-positioned for day trips into the desert, the mountains and across the Mexican border. Tijuana, twenty minutes south, has reinvented itself in the past decade with a serious craft beer scene, an excellent Caesar salad (invented at Caesar's Restaurant on Avenida Revolución in 1924), and a wave of confident young chefs. Cross on foot at San Ysidro and walk in; bring your passport for the return.
Anza-Borrego Desert State Park, two hours east, is the largest state park in California — slot canyons, palm oases, badlands and (in good years) a spectacular spring wildflower bloom. Julian, en route, is a small mountain town famous for apple pie. Joshua Tree National Park is a longer haul — about three hours — but doable as a long day; the surreal rock formations and desert plant life justify it.
Disneyland in Anaheim is 90 minutes north on a good traffic day. Temecula Valley wine country, an hour northeast, has more than 40 wineries spread across rolling hills.