And a carwash is a good idea, the fun part is finding a location at which to host it.
Locations are easier than you think. How many times has someone seen a fund-raiser and/or charity car wash taking place in the parking lot of a gas station or a mall?
Get friendly with the right business owner or two, and problem solved. Canadian Tire stores in particular seem to like showing themselves off as being involved in the community, so they make a good target for something like this.
Fire departments have also been known to open up a hydrant as a water supply at times. When I was a young 'un in Boy Scouts, that was generally how we got our water whenever we did a car wash. I even remember someone gluing together an inverted-U of PVC piping, drilling holes in it, and having a drive-thru pre-wetting/rinse.