Wow, there's a lot you can do in L.A. First of all, not sure if Morongo is the best Casino. IMHO, I think if you stopped off at Pachanga you'll have a better time. It's 10x bigger and has more games to play. Unforntunately, it's no where close to Morongo, so I'm not sure if your travels will take you that way.
As far as L.A., there are tourist things to do and then there are the "city regular" things to do.
Tourist attractions:
Disneyland, Magic Mountain, Hollywood Blvd (Chinese Mann theatre, Hollywood stars walk, Hollywood and Highland shopping center, etc.), Santa Monica Pier, and of course, the beach!
Other "city regular" attractions:
Restaurants - AOC, Jiraffe, Little Red Door (romantic spot), Chaya, Crustacean, Spagos, Mortons, Ruth's Chris, Lawry's, ... the list can go on forever.
Clubs - LAX, Basque, Vanguard, White Lotus, etc.
Malls - 3rd Street Promenade (outdoor shopping), Beverly Center, South Coast Plaza (Irvine), Melrose (eclectic shopping district), etc.
While you're here, if you're a fan of a show, try to see if you can get tickets to a live filming. I think Jay Leno and Jimmy Kimmel tickets are easy to get. Or, if you're a Dodger fan, catch a Dodger game.
Or you can go to the Laugh Factory for some stand up comedy acts, or check out what shows are going on at the Pantages theatre.