The In n Out burger on Sepulveda that FT'ers love is also very close to many of the rental car facilities, so you can always swing by on your way to return the car (if you haven't already stuffed yourself at a taco stand or the temple of some famous chef). The shuttle is great for a 3 hour layover, but you might as well wander further afield with the amount of time you have.

Instead of north to Venice Beach and Santa Monica, you could always go south to Redondo Beach and hang out at Tony's on the Redondo Pier.