Great Falls Park, Holly
The Maryland side has an old tavern as a visitors center, the towpath for easy strolling along the canal, and a boardwalk for an easy but longer walk over to the river. Probably won't be too crowded on MLK day holiday, not as much as summer anyway. National park, Golden Age pass, oh yeah!!
VA side is more a state park like with great view overlooking main falls close by parking, ample parking, and more rugged (not really rugged so much as just natural terrain) along the river.
VA side, the view is spectacular looking right upstream from below. MD side is nice but you are looking more sideways at the falls. But you do get to walk the boardwalk right across a roaring side channel.
Then there's Cunnungham Falls in Thurmont, or Swallow Falls that I have never been to, somewhere out west MD.
