If you combine points of interest that are close to each other into a single trip, you can spend more time having fun and less time in the van.
Winning pairs we have discovered:
1) SYLMAR: the Nethercutt and the L.A. city pool 5 minutes away (has a really great shower/splashpad/kid area)
2) SAN PEDRO: Cabrillo Marine Aquarium and the Pt. Fermin Lighthouse. Check the website for tour days and hours. You could also match kite flying by the Korean Friendship Bell with either of those destinations.
3) NEWPORT BEACH: West Newport Park (and beach) in Newport Beach with the Environmental Nature Center of Newport Beach.
4) VENICE BEACH: before 11 a.m. on Fridays, you can combine walking the canals and visiting the Linney Canal Park there with the Farmer's Market directly north.
5) OXNARD: you could combine the beach with the Gullwings Museum, the Maritime Museum of Ventura County (which recently moved and is free the third Thursday of every month) or the Carnegie Museum of Art or the McGrath Family Farm.