Fondue at Melting Pot or self grillling at Gyukaku can be fun, but I doubt you want your prom clothes (owned or rented) to smell a little like the meal afterwards.
For places with a nice bustling atmosphere, try Bar Celona for Spanish tapas, Mi Piace for Italian, Mojitos for Cuban-fusion, Saladang/Saladang Song for Thai, Crepevine for French bistro or Thaitalian for .. well... look at the name ;)
For a quieter experience, you can try Sushi Roku, Cafe Santorini or JJs. Or McCormick Schmicks / Ruth Chris across from the Paseo -- which is close to Old Town.
Does it have to be in Old Town? If so, best bets might be Twin Palms or JJ Steakhouse, neither of which is all that great, but they are actually in Old Town.
If you can stray outside the friendly confines of Colorado Blvd., I would suggest the Dining Room at the Langham (nee Ritz Carlton).
Other places outside, but close to, Old Town to consider would be Arroyo Chophouse, Madaleine's, or Parkway Grill.
For prom eh ... then I assume you want something fancy and kinda romantic?
Maybe Italian? Like Mi Piace? There's also good fancy chinese at Yujean Kang's
The Melting Pot would be fun? Or Sushi Roku ..
(there isn't anything I really LOVE in old town Pas ...)
Does it have to be in old town? Otherwise there's Maison Akira, Bistro 45 and the Raymond...