fun relaxing drinks and dinner
Might consider Sette Osteria at Connecticut and maybe, is it R? There are lots of good restaurants on P St on the west side of Dupont Circle, but others could make better specific suggestions than me -- or you could just walk that way. If you keep walking, you end up in Georgetown, which would be fun too.
The U Street idea is good, also. I don't know if Busboys and Poets is relaxing, but would be fun (that's also up in the U St area).
1666 Connecticut Ave NW, Washington, DC 20009
Cafe St Ex
1847 14th St NW, Washington, DC 20009
For relaxing I think you might want to head over towards 14th and U, lots of good more relaxing options there. Cafe St. Ex, Coppi's, I forget the names of other places there, help someone? I think there is a place right beside Coppi's that is really good too, but I forget the name.