I stayed at the HI backpackers hostel in 2nd Street, Santa Monica in 1995 and again in 1997. On one occasion I hired a bicycle and rode into Venice Beach to the south, then up to Malibu, a few miles to the north. Great stuff.
I also watched a movie being shot on the beach, close to the pier.
I find the beach way too big at SM. If you're looking for a nice beach day, drive up PCH a bit - there's lots of nice little beaches between SM and Malibu (Malibu has a great beach too). Oh, just be prepared for very cold water if you go into the ocean!