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Anyone remember Appalachian Outfitters in Oakton? A great local gear store, but they went out of business several years ago and they tore down the building (actually, the old schoolhouse part of it was moved down Hunter Mill Road) and put up a Chevy Chase Bank. Boo...
We spent a lot of money at Appalachian Outfitters in the 1980s when my brother and I were Boy Scouts and our father was the Scoutmaster. We also camped on family vacations (example, in 1986 we drove to Ottawa, spent three days or so camping at Gatineau Park nearby in Quebec, then after the time there we drove to Cochrane, Ontario, and spend three days camping in a provincial park nearby and took the train up to Moosonee, etc.), so we had quite a bit of camping equipment. Hudson Bay/Hudson Trail were decent as well, but after a couple of times when the service at the Fairfax Circle location was lacking we started going to Appalachian Outfitters and we found them more helpful. I think their selection tended to include an interesting variety of stuff, too. My old INTERNAL-frame backpack was purchased there and is probably still in my parents' basement or garage somewhere. As an independent store their merchandise often cost a bit more, but it was quality stuff that lasted forever.
Back then the only REI in the DC area was the one near College Park and so there had to be a compelling reason to go over there. Of course, the time my father went hog-wild buying stuff was on a Boy Scout trip to the 1989 Canadian Jamboree on PEI. We drove and obviously that route takes you past Freeport, Maine. Way too much time and money were spent at LL Bean that day when we needed to get moving to reach Moncton that night!
Can anyone recommend a really easy hike (I have a 15 month old who insists on walking, but I'll be taking a stroller too. There will also be a pregnant wife in tow), that makes you feel like you're out of the city/surburbia, i.e. not the mount vernon trail beside a parkway?
When it came time to do the 20-mile hike for the hiking merit badge we did 25 miles on the C&O Canal towpath. Might be a nice option for you because the path is isolated, yet it's also easy to reach. My suggestion is that if you want to do more than a very minimal distance you consider having your wife drive one car and you drive another (if you have that option). Park one car at one end, then everyone rides in one car to the other end. Afterwards you drive back and retrieve the first car. This may be overkill with a 15-month-old and a pregnant wife, though, because my gut tells me you might not go far enough to make it worthwhile.
Another possibility might be to go to Burke Lake Park in Fairfax County and walk around the lake. The north/east side seems isolated, though the south/west side is busier and is near the mini-train so you'll hear its whistle periodically.