Just an FYI - Rosslyn is dead at night. I am a local and there really isn't anything to do after the sun goes down - Rosslyn's attraction is its location: 1) one metro stop to DC; 2) 4 stops to Metro Center; 3) A cheap late night taxi ride home from DC ; and 4) walking distance to G'town. Basically, it's not DC, but as close as you can get
The Palomar seems nice but the rooms are a bit small (visited a colleague staying there a few weeks ago).
The Grand Hyatt is a decent hotel and would be nice for a few nights.
If you are looking to do non-DC related things stay at one of the Hyatts out near Dulles (probably will be more cost effective assuming you have a car???). You could visit Bull Run/Manassas battle field and the Air and Space Annex - maybe even the Ferrari dealership off of Route 28
Cheers