If your anniversary hasn't passed yet, I recommend 2941. My cousin had his wedding there. The restaurant has a fantastic view along the side. Around this time of year though, it might be better to have a lunch there to enjoy the view in the daytime.
As for restaurants...there are a lot of fantastic places - if you want something in alexandria check out ray's the steaks - hands down the best steak i've had. It's not as expensive as mortons or palm or capital ale house...but its dang delicious!
Also in RTC my favorite date place is Passion Fish followed closely by Il Fornaio, first is fantastic fresh seafood, the second is great italian, fresh pastas and pizzas absolutely great flavors.
If you want to venture out into the country, Tuskies in leesburg is good. They also own Magnolias in Purcelville and Fireworks which is next to Tuskies. All have a great selection of micro brews and good food. Fireworks is the cheaper of the three and has great pizza. If you want German fare, across the street from Tuskies and FW is the Hamburg Bistro used to be the Doner. It has great German beer and the beer cycles through fairly quickly as we have a lot of Germans working at their Liaison office in Reston and at the German Military at Dulles.