Although Five Guys has several locations and makes a pretty good burger, I’ll recommend The Rocket Grill on King Street in Old Town Alexandria. Burgers are tasty and the fries are awesome.
Tony Cheng’s next to MCI Center for chinese. Go to the upstairs portion of the restaurant (I believe dosnstairs near entrance is primarily dim-sun).
For great views of DC at night, the Washington Hotel. However, I have never been a fan of their food nor service. Perhaps a cocktail? Also, Old Ebbit Grill is hit-or-miss. Sunday brunch is OK, but lunch and dinner with the tourists can be a drag. If you do go, ask to sit "inside", not out in the atrium.