the diner has a great atmosphere but is really overrated food-wise, i think. not bad food, but not great typical diner food.
tryst is annoying because all of the hipsters with their wireless internet take up all of the good spots on the couches and/or stare you down while you crawl over chairs to get to the one table in the back that hasn't been taken. good chai though, and it's fun to sit there with friends if there is enough room.
there's a fun little place in arlington called common grounds that is a nice relaxed coffee place/bakery-ish hangout. close to clarendon and an interesting record store - orpheus.
hope your birthday is fun. if you don't have high expectations you won't be let down! but don't feel pressured to make it like the best day, ever. go skydiving.