Traveling to Los Angeles without a car?

I'm planning a little vacation to LA by myself in Sept and was debating whether to rent a car or not. I'm only going for 4 days and I want to stay near LAX or at a hotel near everything that has shuttle service to LAX. I want to do some sightseeing and spend some time at the beach (santa monica). Can I get around easily without a car? Is the public transportation decent? I'm going Tues--Fri if that makes a difference...