You can rent tubes at several spots on the Pemigewasset - one being ski fanatics (
http://www.skifanatics.net/ ) for a leisurely float down the river. We did it two summers ago and it was really a nice experience. Supposedly you can stop wherever and eat your picnic lunch etc, however, it had rained heavily the day before so the river was moving swiftly and many of the "rock islands" were underwater!!