Every nature center--MD, DC, VA pretty much has trails surrounding it. Some of them could be more clearly marked than others.

This thread also had some good suggestions (although it was for a younger kid):
http://www.dcurbanmom.com/jforum/posts/list/15/293701.page

Sugar loaf Mountain on MD might be within an hour drive, depending on where in DC you are. It can get busy, so go early.

Burke Lake in Springfield/Lorton is nice and very flat if your kids complain easily. In the spring they also have a train which your 5 year old might enjoy.

Another poster mentioned Prince William Forest National park, and I agree.

It is rarely crowded there---I used to hike it at least once a week during the summer and I would literally go hours on a Saturday morning without seeing anyone else. I saw lots of animals there, including a beaver that chased me!

You mentioned that you wanted it to keep you occupied for half the day--so if you want something else to occupy the other half, it is just down the street from the Marine corps museum. http://www.usmcmuseum.com/index.asp
This is a GREAT free museum with plenty of free parking.

If the weather is nice bring a picnic--there are picnic areas at the National park and also near the parking lot of the Marine Corps museum.

13:17 - one more - the Fairfax County Connector Trail is fun. I like to park at the Oak Marr Rec Center and pick it up there.

There are several places I love in Arlington. Roosevelt Island is great. The Potomac Heritage Trail is cool - you can pick it up by Roosevelt Island, by Windy Run, by Donaldson Run or Chain Bridge. (All in North Arlington.) I've taken my 5-year-old on hikes at all these parks - she especially loves walking on rocks to ford tiny streams, like you can do at Windy Run and Donaldson Run.

In the city, I like the Glover-Archibold Trail (pick it up where M St. becomes Foxhall, or at Van Ness St.) or Rock Creek Park. I also like Sugarloaf Mountain if you're willing to drive for an hour.

Turkey Run is a nice getaway without really going anywhere. You can walk in the woods and along the river and feel far away from the city. It's our favorite hike when we want to get outside, but are stuck in town. It is much quieter than Great Falls.

If you are willing to drive out I-66 an hour (depending on traffic), Sky Meadows State Park is great. The trails are fun, and there are often history or nature programs on the weekends.

There is a 3 mile trail that starts at the top of the stables in Rock Creek Park (near the Nature Center) and winds along Rock Creek in a bouldery beachy section opposite Beach Drive. It's downhill, flat and then uphill but even kids can do it. Best days to do it are when Beach Drive is closed to traffic, it's quieter. Keep an eye out for the uphill shortcut on the right.
There is a flat trail near George Mason's historic home in Mason Neck Va. that leads out to the Potomac and back again, it would be a good two-fer with lunch in Occoquan and maybe peek through the fence at the Pirate's Cove waterpark.
Cunningham Falls is a rewarding walk but you have to drive to Thurmont so it's further than you said you wanted to go.
Great Falls with little kids is scary but the MD side is OK if they can be trusted not to scamper/climb away.

There are easy hiking trails at the national park in Greenbelt or the Prince William National Park. We have also gone hiking (very easy hiking--more like nice 45 minute walks in the woods) and car camping at Watkins Regional Park in Bowie, MD. In addition to hiking, there is a great playground and nature center there that is open year round, and in the summer there is a "farm" with animals, train and carousel.

From Capitol Hill, it takes about a half hour to get to any of these places.

There are parts of the billy goat trail that I don't think are good for little kids. It is rather like mountain climbing in sections.

Roosevelt Island (don't go if it has rained a lot because of mud)

Riverbend Park (similar mud problem)

Nature centers like Hidden Oak and Long Branch

Growing up in the area I loved the billy goat trail, most of the trails in rock creek park, and sugarloaf mountain.

Great Falls. Might be pretty easy for your kids' ages but really nice views of the water. My 3 year old loves it. There are also "ruins" from the canal that are fun places to explore and have a packed lunch.

Also on the easy side is the trail behind the Nature Center in Rock Creek Park. The center itself is small but there is a small planetarium and many taxidermied animals (again, your kids might be totally unimpressed). There is a loop trail behind the center that's about 1.5 mi. Then, you can walk over to the stables and visit the very friendly horses at the riding school.

billy goat trail- Great Falls MD side

Please recommend your favorite family hiking trails. Kids are 5 and 7, so nothing crazy-strenuous, just enough to keep us out for half a day. We'd prefer to stay within an hour of DC so we're not driving longer than we're hiking!
Thanks