It really all depends on what you consider "fun" as there is a ton of stuff within a reasonable drive or train ride:

-Annapolis is always nice, especially if you can get a place to stay on or near the waterfront. The Paca House and Gardens is a nice way to spend a half day; they have a really massive koi pond in the gardens and it's a mellow, relaxing place

--Charlestown Races and Slots if you want to whoop it up and gamble but don't want to go all the way to Atlantic City (plus Charles Town either just got, or is getting ready to get, table games in addition to their slots)

--There are several nice day spas in/around Berkley Springs, West Va

--There are some wineries in Frederick County, MD and a bunch more along Route 15 in VA right along the Potomac so you could make a one-night itinerary of winery touring in the daytime and overnight in a B&B (not many decent, proper hotels up this way so B&B's will be the better choice)

--If you are outdoorsy, you could go out to Catoctin State Park which is up near the MD-PA line; gorgeous, especially this time of year with the leaves turning. I love taking my motorcycle up there. Really one of the most beautiful places in the area.

--Amtrak Train to Philly would be about an hour or so, and another hour or so to Manhattan (sorry if my times are a little off, as for me time stands still when I'm on the Amtrak... meaning it crawls so slow....)

--Amtrak or Marc (if it's running) down to DC is under an hour

--Oh and in North Beach, MD, they have a big pre-Christmas celebration where Santa comes in on a speedboat. It's been a long time since I attended but I remember it to always be good fun.

Post a little more about what you're looking for and I'm sure you'll end up with a ton of suggestions!