TunaTown or in Huntington Beach or anywhere else its fun to celebrate a 21st birthday!
You'll probably have a hard time finding "great food" and a "fun bar" on Main Street. I've Never been to Tuna Town, so I can't speak about that. But you could check out Sharkeez (food's not very good, but a good bar for a 21st) or 301 Beachfront (food's better than Sharkeez, nice lounge area, but not quite as big of crowd at the bar). Fred's is okay too, usually crowded. There's also Hurricanes, but I've never eaten there and they often charge a cover. You can also check out Huntington Beach Brew Co (they make their own beer and the good is alright) and Aloha Grille (pretty decent Hawaiian food).