Kind of depends on your weather that day, but here are a few options if you opt to hang around the coastline:

1. Whale Watching - I think it's just the beginning of the migration season for northbound whales. The Point Vicente Interpretive Center has some information(http://www.acs-la.org/GWCensus.htm) and offers great views, even if you don't see any whales. The drive around the Palos Verdes Peninsula is visually stunning on a clear day

2. USS IOWA - Historic Battleship, very cool to visit: http://www.pacificbattleship.com/

3. Long Beach - there's the Aquarium of the Pacific (http://www.aquariumofpacific.org/) and The Queen Mary (http://www.queenmary.com/)

I'll let other chime in on the Orange County options