Rabbit Peak via Villager Peak in the Anza-Borrego Desert is considered by many to be the most challenging day hikes in San Diego County. Most do this as an overnight trip. Villager Peak is renowned for its breath-taking sunrises over the Salton Sea.
The route starts from the north shoulder of the Borrego Salton Seaway. You'll climb up rugged terrain about 4,000ft to Villager Peak. Then follow the the narrow ridgeline to Rabbit Peak. This undulating path will add about another 2,000ft of vertical. Now follow the route back which adds about another 2,000ft of elevation gain to your car.
It requires experience navigating faint or non-existent trails. A GPS device that functions offline is highly recommended for navigation. Be prepared for a everything from scorching heat on the desert floor to below freezing temperatures on the summits.
Modern Hiker provides a detailed description of this route: https://modernhiker.com/hike/hiking-rabbit-peak-via-villager-peak/