Depends on the budget I think
For a casual birthday dinner with good food and drinks that’s not too fancy and won’t break the bank: Monarch 9 in YL, or KIN Izakaya in Fountain Valley
If we’re looking to go nicer: Summit House, Tsuruhashi, or Ohshima for their omakase
If money is not a concern: Mastro’s, Knife Pleat
Drake, Brussels, Broadway, 230 Forest, Nicks/South of Nick's, La Sirena.
Spend some time just getting lost at Main Beach, Forest/Ocean Ave, stroll up to Heisler Park and check out all of the local beach coves - Fisherman's, Crescent, Shaw's, Diver's. or head south to Moss, TableRock, Thalia, etc.…