There are hundreds of hiking trails in the local back country of San Diego County.
The following are some of the local trails to explore, depending on how much time you have here:
Anza Borrego Desert State Park
Cleveland National Forest
Cuyamaca Rancho State Park
Hikes along the Pacific Crest Trail (this is the trial that runs from Mexico to Canada that is well traveled during Spring and Summer)
Laguna Mountain Recreation Area
Lake Cuyamaca Recreation & Park
Santa Ysabel Nature Center
Santa Ysabel Preserves
Volcan Mountain Wilderness Preserve
William Heise Park