A hike to Great Falls is a good thing to do. You could of course just park at the falls and walk over in 10 minutes, or you could make it a fairly strenuous hike if you go on the Billy Goat trail a bit below the falls on the Maryland side. The views of the falls are arguably better from the observation decks on the Virginia side, but the hike is better on the Maryland side.
You can also park at Fletcher's Cove and rent a bike or a canoe or a row boat or a kayak for a paddle either up on the C&O Canal along the Potomac River or paddle on the river. Paddling up from Fletcher's takes you to below Little Falls. If you are into easy whitewater, you can paddle areas below Great Falls: either paddle upriver through Mather Gorge all the way to the falls from say Angler's Inn put in (with no or possibly some very short portages, depending on your boat and skills). Or go up the canal then float down the river via Mather Gorge. Or paddle down from Angler's to say Lock 10 - a very nice and quiet area with class I-II water in low levels, could be class 3 or more at higher levels.
You could put in at Columbia Island marina across from the monuments (just off the Pentagon) and take a flat water trip to see the monuments and the area from the water. You could paddle down to Mount Vernon if you like (it is a long paddle from Columbia, but you can put in down at one of the other marinas closer to Mt. Vernon).
On the Chesapeake Bay you could explore Kent Island on the Eastern shore or see the Thomas Point Lighthouse from the water on the Western shore off Annapolis.
Etc. Lots of places to check pout. As I said above, post at CPA if you want to find some company. There are also some Meetup groups that partially have overlapping membership with CPA but sometimes have group paddles that are not listed on the CPA calendar. CPA has "pirate" groups at most locations going out on specific evenings each week - you can attach to one of these groups if you like.
Paddler's Truck Rack
Kayak & Canoe Covers
|Table of Contents|