instrumentally on 8/18/2014 10:47:17 AM:
Great Falls (Tavern) is roughly 15 miles from the end of the C&O canal/trail. I don't know about multi-day parking at this site, however. It costs $5 to enter the park (unless you arrive a few hours before closing, at which point entrance is free). Contact the Tavern Visitor Center to see what the policy is on overnight/multi-day parking: 301-767-3714
There are several commercial parking garages at mile point 0. To find them, use Google maps, zoom in to a half mile around MP0, then type PARKING in the search field. Each resulting pinpoint can be clicked and phone numbers will be displayed which you can use to call the establishments and see what their policy is with regards to multi-day parking.