If you can get into the space center before close they may stop entry at 5:30 they MAY still allow people inside to finish their tour and maybe actually close the doors at 7.....something to look into.

I agree with Eric...Reston town center is more manageable and there are a bunch of restaurants.

To get to DC and come back can be a bear. At night the monuments of the white house Washington monument and Capitol are lit up so you can do a driving tour. It's a half hr drive with no traffic.

Another option is Tyson's corner where there are two separate malls with restaurants. Off mall property there are a bunch of restaurants as well. It's about 15-20 min from the airport