San Diego is probably the best city in America for making a quick trip to Mexico. Tijuana, San Diego's twin city across the border, is only a short trip by car. The San Diego Trolley also provides service from downtown San Diego to the US-Mexican border. Avoid driving hassles and long waits when returning by parking in pay lots near the border and walking across. Taxis, buses, and private car hires are all available. if traveling to Tijuana Airport, Mexican airline Volaris operates a bus service between that airport and San Diego's Santa Fe Train Depot.
For a delightful, low-key alternative, drive 60 minutes on the American side to the small border crossing of Tecate (home of the Tecate brewery). It's a short walk to the town square. Coming back, there are typically only a couple of people in line at the pedestrian crossing. You can easily combine a trip to the train museum in nearby Campo with a quick trip across the border for lunch!
Julian is the largest and most popular mountain community in San Diego County. Also, nearby is Cuyamaca Rancho State Park and Palomar Mountain which has a large observatory. Beware, on hot summer days, the mountains are actually warmer than the city (as they're next to the desert).
The greater San Diego County has a lot of smaller, more private beaches, and some great small towns to stay in and explore. Further east, the Imperial Valley and the California Desert give a change of scenery.
There are no boats to Catalina Island (Avalon) within San Diego County. You'll have to go north into neighboring Orange County to the pier at Dana Point. By car, take I-5 to exit #79 Pacific Coast Hwy 1 (make reservations).
Temecula Wine Country is located about 60 minutes northeast of San Diego and makes a good day trip. There are about 20 vineyards (with tasting rooms) located fairly close to each other. One hour further is the mountain resort of Idyllwild which features shopping and outdoor activities in an alpine forest.
