Riverbend Park, located just over a mile from the class VI rapids at Great Falls offers maybe a bit less spectacular whitewater, but unless you are an accomplished white water kayaker, this will be plenty.
Most of the area is very rocky. If the water level is too high, boat launching is prohibited. Once I have arrived after a 1.5-hour downriver trip from Algonkian Park and at Riverbend found a sign proclaiming that boat launching was not permitted, because of high water. The park manager is very nice, knowledgeable about the river and helpful. You might want to have a chat with him, before setting out for the first time. In any case, water levels will vary quite widely. When they are low, you can expect to hit quite a few rocks, no matter how hard you try to avoid them. That shouldn't be a problem with a plastic boat, though.
Upstream from Riverbend the Potomac has a great many islands, lots of birds of all descriptions and I and II class rapids, all of which a person of reasonable strength can paddle in either direction. Although strenuous, there should be no trouble going some 4, or 5 miles upriver in an afternoon. Any farther and you will be hitting the Seneca breaks, which are much more fun to ride one-way - downstream. In any case, a good, 1.5 - 2 hour paddle will get you as far as one of the unnamed and largely undeveloped parks, at the end of Seneca road in Great Falls, close to the Seneca Breaks and at the lower end of the whitewater of the GW Canal on the Virginia side of the Potomac. Considering that you have paddled upstream, this could be a good turning point and time to finally enjoy the payback of floating downstream, watching the wildlife and enjoying the quiet.
Riverbend charges $2 for launching a boat.
Riverbend Park may be reached by taking Beltway Exit 44 which is Georgetown Pike west to a right on Riverbend Road. From Riverbend Road turn right on Jeffery Road and follow for approximately 1 mile to the park entrance.