It's much easier to stay in the VA suburbs of Arlington - close to Metro for day trips. Greenbelt is pretty isolated and no place handy to eat. You can buy Metro week passes -- check the website www.wmata.com. There are also special day passes. Don't drive into the city! Poor parking opportunities & traffic.
Also, please remember that July is is REALLY HOT - 90F usually with afternoon thunderstorms. This has wiped out our English visitors for a day or 2 on arrival -- start early in the day.

I would respectfully disagree with other posters here and suggest that you DO stay in the city. Unless you will be driving in from Dulles between 6:30-9:30 am on a weekday it should be an easy, pleasant drive (Route 66 is a multi-lane expressway that takes you straight into the city). Grab that place in Dupont Circle and just park the car somewhere cheap. You won't need it to get around the city, in fact it could be a hindrance.

RE jnn1964's post -

We have Hilton Arlington and Towers (Balston Metro stop below hotel)May 21 - 24 for $36.00 per night - through Priceline.com. Don't know if you are familiar with Priceline or can use it with a credit card not from North America.

Have never been to DC. Looking forward to my visit.

Have a wonderful time.

Zig

cheers all for the great advice ........ ive certainly got my work cutout for me researching the various places so thanks once again to all who contributed and ill let you know how we got on come september