A popular activity when visiting LA is to go to a TV show live taping. There are over 100 live television show taped in the LA area and some pre-planning before your trip could give you the chance to be a part of the audience.
The Tonight Show With Jay Leno
Jimmy Kimmel Live
The Ellen Show
TV Taping Ticket Agencies:
1iota - http://1iota.com/Tickets
Audiences Unlimited - www.tvtickets.com
On Camera Audiences - www.on-camera-audiences.com