The Sebago Lake area has a ton of nice campgrounds. I have a seasonal site at Lakeside Pines in Bridgton, right on Long Lake. It's a very nice campground. Beach, boat ramp, rental docks ( if you need them), activities, etc.
http://www.lakesidepinescamping.com/
Next is Point Sebago. Also very nice they have tons of stuff there on one of the largest lakes in the area.
http://www.pointsebago.com/
Another one on Sebago lake is Sebago Lake family campground. small, but clean, at least last time I trolled the area.
http://www.sebagolakecamping.com/
Crystal lake is a nice lake just on the other side of Long Lake. We just looked at this campground for some friends. It was great! They even have boat rentals.
http://www.vacationlandcampground.com/home.html
All of these places have sites right on the water, if you can get them. Good luck!!