I'd stay at The Shade Hotel in Manhattan Beach. If you do, eat at the Strand House, which is only a couple of blocks from The Shade. The hotel loans you beach comber bikes and you can ride north and/or south along the strand to other beach towns, easy, flat rides and they aren't too far apart. As far as the Boxster, not certain when you say "test out a 981" if you are going for a dealer test ride or renting one. If renting one and you have the time, drive to Malibu and then pick up Mulholland Drive back to the 405 essentially at Beverly Hills. This takes you through the Santa Monica Mountains. If possible, due this during the week. If staying more towards LA, then you'll take the 10 to Santa Monica to get to Malibu. If you do this, you have to hit Venice/Muscle Beach. Quinessential LA beach scene with lots of "street" performers and great people watching, though, with the medical marijuanna dispenseries, it may be a bit more seedy then when I did it a long time ago. It is really close to Santa Monica, which you should do and hit the pier there. Nice shopping and cafes to refuel yourself before heading towards Malibu. The PCA can be very heavy traffic wise, but with the top down, the sun shinning, does it really matter as you'll soon hit Mulholland and the mountains. I'd also suggest stopping on Rodeo Drive if you do the Mullholland as it takes you back towards Beverly Hills. The famous area is only a couple of blocks long, but you've seen it in countless movies. If the Dodgers are in town (seasnon begins Monday), you have to go to Dodger Stadium. If you want to check out the Hollywood sign, be careful on your Internet search. A lot of locals who live near it have put bogus directions up to keep the tourists out of their neighborhoods. Not sure if it is still the case, but there's another famous, more funky than shiek, shopping district on Melrose Ave. Catch a movie at Grauman's Chinese Theater, or at least check out the cement hands. It's not too far from Melorose Ave. Griffin Observatory has some great views of LA, but you need to catch it on a clear day. I haven't done this, but the Getty Museum is supposed to be spectacular, if you are into museums. If you are into animal rights and care about the treatment of farm animals, you can go to the Farm Sanctuary, which is north and east of LA, only about 45 mins in a town called Acton. This is the smallest of the three farm sanctuaries, but they do provide tours. Have a blast. If CA ever gets its act together financially and stops taxing the heck out of its citizens, in ten or twelve years, i may just retire in Manhattan Beach or Santa Monica.
If you like movies, one other thought is to do a topdown drive along Sunset Blvd. This is where all the movies adverstise on billboards. Going topless will add to the visual experience. Traffic is heavy, but since you'll be gaucking, that's all the better.