What are your favorite hiking trails in the DC area?

The closest and easiest hike for DC (and probably the most popular) is the http://www.midatlantichikes.com/id163.html along the Potomac river, which is not too difficult or strenuous and is a good casual day hike. A little further out by Frederick MD is http://www.sugarloafmd.com/sl_trails.html, which is more of a glorified hill than a mountain but still has some nice views of the DC area. If you're OK with driving a bit more there's Skyline Drive in Shenandoah National Park which has some great hikes and has trails that are part of the Appalachian Trail- I've hiked and can recommend the http://www.hikingupward.com/SNP/CorbinCabin/ as well as staying in Corbin Cabin if you want to make a 2 day thing out of it.

http://www.localhikes.com/Hikes/Old_Rag_Mt_0000.asp is probably the most fun hike in the area. It's the hike that got me into hiking. It's about a two hour drive away in the Shenandoahs. The last half mile or so to the top is all rock scrambling. It's challenging, but very rewarding with GREAT views at the top. I've done it with inexperienced hikers and it is totally doable with minimal experience.

There are miles and miles of trails through the http://www.nps.gov/prwi/ in Dumfries. A little farther south, the http://www.nps.gov/frsp/ have extensive trail systems that are generally well marked, and as a bonus, are full of history.

The http://jwbasecamp.com/maps/BRO/index.html, which goes between Bull Run Regional Park (just south of Centerville) and Fountainhead Regional Park (on the Fairfax County side of the Occoquan River west of Lorton) is very nice along its whole length. It's pretty hilly in the eastern/southern 2/3rds, but it's quite pretty. The http://www.fairfaxcounty.gov/parks/cct/ is a nice place to do slightly longer hikes since it goes past porta potties and rec centers every 5-10 miles and goes from Lorton up to Reston. The CCT is a mixture of dirt, gravel, and paved trails. I enjoy walking along the http://www.nps.gov/choh/, you can start any where from Georgetown through Great Falls and points north depending on what you want to see. The Mount Vernon Trail, which starts at Mount Vernon and goes up to Rosslyn is also a fun (if paved) hike. North of Rosslyn you can continue on Both the Mt Vernon and Canal can be pretty busy and crowded with bicyclists near DC if that's a concern during the weekend.

I came in here to say Old Rag. It's not the easiest hike in the world, but it's hardly difficult. The trail starts off switchbacking through the forest, then pops up onto the ridgeline, where the exposed rock leads to some minor scrambling as noted by havemercy. Another part of the fun (for me at least) is that once you gain the ridgeline, you come across a number of "false summits" where you'll think you're near the top, only to get to the top of the knob you're climbing and see another stretch of ridgeline rising gradually ahead. I had that happen at least 3 times when I went, but it may have been compounded by the fact that there were low clouds that we were basically hiking inside of.

Both of my favorite place have draws for history buffs. When I lived in Germantown, MD, my favorite spot was the part of the C&O canal with the http://www.riverexplorer.com/details.php4?id=637. Going to http://www.livestrong.com/article/151986-guide-to-hiking-trails-near-harpers-ferry-west-virginia/ is fun too. The C&O and the Appalachian Trail can both be accessed from there.

http://www.nps.gov/gree/ has about 15 miles of pretty variable terrain and nice woods, if that's your thing.

Here's a http://ask.metafilter.com/159973/I-Long-to-See-Your-Smiling-Valley. George Washington Forest is out on the West Virginia border, but it takes about as much time to get there as most of the Shenandoah trails, since more of the mileage is on large highways. I'd recommend the http://www.hikingupward.com/GWNF/WhiteRocks/ and http://www.hikingupward.com/GWNF/LittleSchloss/ trails out there. If you're hiking with a dog, it's particularly nice since you can go leash-free. I love taking http://www.hikingupward.com/SNP/LittleDevilsStairs/ in Shenandoah - the two miles up are tough, but fun, and then it's an easy decline down that's great for chatting with friends. Plus there's a nice spooky graveyard along the way! Closer in you have http://www.hikingupward.com/OVH/ManassasBullRun/index.asp, which you can roam around in. It can get hot in the summer, with all of the open meadows, however, and ticks are not uncommon this time of year.

Late to the party, but the C&O Canal trail is good for endurance training. You can pick it up anywhere in Georgetown, and it could lead you as far as Pittsburgh, I think, if you stay on it long enough. There's the new Potomac Heritage Trail, which is pretty extensive, but there are doable short day hikes. I've hiked the section from the Key Bridge up the the Chain Bridge, then back down the C&O towpath in a couple hours. It's very rocky on the Virginia side and there are no facilities anywhere along the trail. There are three legs of the Billy Goat Trail, if you haven't already discovered. One of them -I think it's A- is pretty hard, with lots and lots and lots of scrabbling over rocks. It's not a simple walk in the woods. They usually have to rescue hikers who underestimated the difficulty several times a year. The other two legs aren't nearly as difficult. You might want to hook up with the http://www.capitalhikingclub.org/home.aspx.