Hey! I live in Western Prince William county!

We have 4 boys between the ages of 4-10.
Are you looking for free/cheap things to do, or just stuff to do in general, even if you have to pay?
I know I've been wanting to check out the naturalist center
http://www.mnh.si.edu/education/fieldtrip/planned_programs/naturalist_center/index.html
Also, check out your local libraries, many have free programs for kids in the summer...in fact, I'm leaving in a few minutes to take my kids to a free craft program.
We are members at the Shenandoah Discovery museum
http://www.discoverymuseum.net/
out in Winchester. They are part of the Association of Childrens museums and also the Association of Science-Technology centers, so they have a recipricol agreement with all of those member museums--we've been able to visit the childrens museum in Baltimore, the one in Richmond, the one in Charlottesville, the Science museum in Richmond, etc. for free! Plus when we've gone on trips out of the area (Philadelphia, Houston, Louisville, etc) we've been able to visit museums there too!
Have you visited the Marine Corps Museum yet?
http://www.usmcmuseum.org/
It is VERY well done, free, plenty of free parking, and has a really nice playground/shaded picnic area outside.
National Parks? We usually have an annual park pass (ours just expired in May, but we will be getting a new one no later than August!) and love to picnic and hike in Prince William Forest and Shenandoah. Also maybe check out Ft. McHenry in Baltimore--we did that one day and then stopped at the children's museum in Baltimore. Around here, there are TONS of National Parks/monuments/etc. so you can get a lot of use out of a annual pass.
Do you like to bike? We love the W &OD trail for family bike rides
http://www.wodfriends.org/
We bring along a picnic lunch or stop at Carolina Brothers BBQ (literally right off the trail)
http://www.carolinabrothers.com/
Um....if I think of more I'll come back and post more!