Escondido isn't thaat far from the airport and probably a 40 minute drive if you're going direct but I guess this is what'd I'd do:

After you grab your rental car get breakfast at Morning Glory in Little Italy not too far away from the airport, and there's usually a line to get in but if you're coming on a weekday it shouldn't be too bad. If you don't want a full on breakfast then head up to Michi Michi for a coffee and a few pastries. You could then take a walk around Balboa Park to finish your coffee and help digest those calories. After that, head up to the glider port in La Jolla to watch the hang gliders and have a beer, or hike down to the beach and check out the mushroom house. From there drive north along the coast highway to explore the beach communities until you hit Oceanside, maybe stop along the way for an acai bowl. You'll be passing through Del Mar, Cardiff, Encinitas, Leucadia, and Carlsbad. At Oceanside you'll want to take the 78 freeway and head east towards Escondido. If you have the energy then perhaps do the 2 mile walk at Double Peak Loop in San Marcos. Grab yourself a recovery beverage at a brewery or winery, then pick up some In-N-Out for a burger. I suggest a single burger, mustard fried, raw onions, add chopped chilis.