I live pretty close to you (I'm on the border between Rockville and Gaithersburg). I made two quick trips to Catoctin/Cunningham Falls this winter. it's a lovely site and I was just about the only one there (which is why I like going places off-season...). I haven't been there when it's warm out, so I don't know what the crowds are like during tourist season.
The falls itself is a very short hike from the parking lot, over easy terrain. It was dicey when snow-covered, but it's cake otherwise. The falls are tricky to shoot, at least in the winter, because there's very little direct light to be had. In the summer, I'd expect to find extremes of dynamic range, so I'd bring ND and polarizing filters and expect to shoot on a tripod. I'd be happy to go out there again if you're interested (anyone else from the DC area around here?).
Very close to Catoctin State Park is also the Catoctin Furnace, which is the ruins of a pig-iron furnace that was used up until about the turn of the 20th century. I was just there a few weeks ago. It's nothing amazing, but it's worth a few minutes if you can get there early or late in the day.
