In addition to what others said, you can get to Rt 16 trailheads for Carter/Moriahs and Great Gulf in about 30 - 40 minutes, Rt 2 trailheads for the southern Mahoosucs in a bit less, and access the northern Mahoosucs by following the Sunday River Rd to the Wright Trailhead. There are many places to hang out by the Sunday River and swim if you don't mind cold water. The further up you go, the better your chances of not getting caught if you "forget your swimsuit".
Also, be aware that Rt 2 is undergoing major work between Bethel and the ME/NH border. Right now only about a 1-km piece around West Bethel is gravel, but you might get held up anywhere along that stretch.
For canoeing, I like the stretch of the Andro from Gilead to Bethel. You can take out at Bethel Outdoor Adventure, but only if you are his customer...he runs shuttles, or just downstream at Davis Park which is on Rt 26 very near its intersection with the Parkway.
Also, that weekend is the Trek Across Maine, so expect very heavy (2000 or so) bicycle traffic from SR ski area along Rt 2 east on that Friday.
That's probably way more than you wanted to know, but if you need more just holler.
vegematic