Twelve miles northeast of downtown San Diego sits the largest urban park in California — and one with an unusual backstory. Mission Trails Regional Park covers 7,220 acres of canyons, peaks and lakes that once made up Camp Elliott, a Marine Corps training ground where tens of thousands of troops prepared for both World Wars. At its height the site held 41 firing ranges. San Diego took over the land and opened the park in 1974; today, 65 miles of trails cross terrain where tanks once rolled. Erosion still occasionally unearths rusty old ordnance, so if you spot something shell-shaped, leave it alone and tell a ranger.

For hikers, the headline attraction is Cowles Mountain, a 3-mile out-and-back with roughly 1,000 feet of climbing to San Diego's highest point. On a clear day the summit views stretch from downtown to Orange County and even Mexico. Trails range from easy creekside strolls to tougher ascents through grassland, woodland and chaparral, and the wildlife list is genuinely wild for a city park — bobcats, mountain lions, coyotes, roadrunners, hawks and owls all live here.

The park's Visitor Center is the launch point for free volunteer-led nature walks, including a loop among Coast Live Oaks and rare Engelmann's Oaks, and a stroll along the Grinding Rocks Trail, an archaeological site where the Indigenous Kumeyaay once ground acorns into a food called shawii with mortar stones. That same area draws rock climbers to the cliffs.

Water lovers get two lakes. Lake Murray, 171 acres, has a 6-mile paved, accessible loop popular with runners, walkers and cyclists, plus kayak and small-boat rentals, picnic tables and barbecues. Swimming is off-limits, but anglers do well — the lake is stocked with largemouth bass, bluegill, channel catfish, black crappie and trout. Over at Kumeyaay Lake there's a campground with 46 tent and RV sites, showers and drinking water, open Friday and Saturday nights only; on weekdays it becomes day-use picnic space.

Best of all, entry is completely free, the park closes only on major holidays, and San Diego's mild weather makes it a year-round outing. It's an easy add-on to any San Diego trip when you want a break from the beaches and Gaslamp Quarter — just bring shoes you don't mind getting dusty.