Ventura has some beautiful beaches, as does Santa Barbara. Santa Barbara also has a beautiful mission and State Street is fun if you like shopping. The Santa Barbara Zoo is a nice smaller zoo. Santa Maria is about 2-2.5 hours north of Simi Valley so unless you're spending a couple days I wouldn't suggest making the drive up there. Check the rocket launch schedule for Vandenberg AFB for the time frame you're planning. You might get lucky and be able to see a launch which is a really cool experience.
The Seabee museum on Port Hueneme is a neat experience for any military/history buff. For info on that go here:
http://www.history.navy.mil/museums/seabee_museum.htm
You can do a Channel Islands boat tour, whale watching tour(depending on the season of course, and check out the butterfly sanctuary in Goleta (
http://www.goletabutterflygrove.com/)(whether you see any butterflies depends on the time of year but it is absolutely beautiful year round).
There's also the Commemorative Air Force in Camarillo, Ca that has a lot of old warbirds in flying condition as well as some being repaired/worked on. It's a very neat museum to visit as well.
http://www.cafsocal.com/
That's all the things I can think of off the top of my head after a long weekend of work. Once I've gotten some sleep I'll probably be able to think of some more things to do.