Long Beach is my second home, where I went to college and resided for four years, and has a special place in my heart. It is also, in my mind, one of the best places in the nation and THE best city in the LA area. It's the most diverse city in the world (USA Today) and it shows!
First of all, if you eat at a major chain restaurant in Long Beach you should be slapped in the face. There is SO much GREAT local cuisine that going suburban is a waste of your time. Check out all the Cambodian/Thai joints, Mexican spots and burger stands on Anaheim Street, many of them open late. For Cambodian, I recommend Siem Reap on Anaheim and Orange, for Thai, Sophy's on Anaheim and Redondo; for a great Quesadilla go to La Concha on Anaheim and Molino, and for a funky burger stand go across the street to Omegas (Anaheim and Ohio). There is also a Tommy's burgers on Anaheim and Ximeno. Furthermore, Long Beach is home to a location of Roscoe's House of Chicken and Waffles, and if you come to LA/LB for any space of time and don't eat Roscoe's, you've wasted your time and money.
Stay at the Queen Mary. Do it. It's a helluva lot of fun, and they have a little "Passport Bus" that will run you from the Queen Mary back into the downtown. If you wanna go out at night you have a couple choices: the higher-end nightclubs and lounges of Pine Street (I recommend Cohibas or the Rythm Lounge), the funky, gay friendly bars of Broadway (between Alamitos and Redondo, generally) or the surfer-dude bars and pubs of Belmont Shore (2nd Street between Livingston and Bay Shore). There's also some bars, like the original Yard House, in Shoreline Village. You can bring your car to the LBC because it's a fairly easy city to drive in, and you'll need it if you want to make excursions into other parts of the LA area, but you dont REALLY need it within the LBC. Cabs are cheap and the city aint all that big.
Oh and if you want to take a great walk, go walk along the beach from Junipero Ave (there's metered parking there) over to the Belmont Pier or even further if you want. It's an incredible sight, especially at night!
And if you want to go do something really locally-flavored that tourists dont really do, head over to PCH and Martin Luther King and go to the VIP Record Store...yes, the one from the Snoop Doggy Dogg videos, the one where Warren G and Snoop used to record songs in the back. THAT VIP Records store. It's in the hood, but generally the staff is friendly and don't care if you're Black, White, Brown or Purple...except the one fat ass security guard who was working there for awhile. Just ignore that guy if he's still there. Also, while you're there, head on over to the West Coast Choppers garage and eat at Jesse James' Cisco Burgers across the street. You'll find them both along West Anaheim Street right before the 710 Freeway.
But no matter how short or long your visit will be, EAT AT ROSCOES!!!!!! DO IT!!!!