Off the beaten path

Here are some places to take the kids that are off the beaten path and are FREE (or very inexpensive)

Franciscan Monastery, Washington, DC
1400 Quincey St., NE,
Washington, DC 20017
http://myfranciscan.org/
great place to tour the gardens and the catacombs.

Planetarium at Rock Creek Park, Washington, DC
http://www.nps.gov/rocr/planyourvisit/planetarium.htm
nature center, trails, hiking

Lake Accotink Park
Springfield, VA
hiking, canoeing, paddle boats, mini golf, merry-go-round, and more...
http://www.fairfaxcounty.gov/parks/accotink/

Arlington National Cemetery
Arlington, VA
just walk around with the kids, work on math skills, visit the Tomb of the Unknowns, watch the changing of the guard.

Potomac Overlook Regional Park
2845 N. Marcey Rd., Arlington, VA
nature center, trails, hiking....

River Farm Park 
Alexandria, VA (off the GW Parkway)
explore the children's garden

Roosevelt Island
off the GW Parkway
hiking & trails