It was only in my second-to-last weekend of living in Boston that I discovered the kayak rentals on the Charles River. Of course, it was incredibly fun, and I left kicking myself for not trying it sooner.
The Charles separates Boston from its northern neighbor Cambridge, home to the Head of the Charles Regatta (one of the world's largest rowing events), and is a fairly popular waterway for boat traffic. Actually, because of the river traffic, I had to pass a quick safety test to learn the rules of the waterway before I was allowed to don my lifejacket and play coxswain. The river itself was serene, and I was mostly kept dry (until the wake of a much faster speed boat caught me!). The best part was paddling under bridges and seeing the Boston skyline from a new perspective.
It's possible to rent from at least five locations along the river (I picked Brighton). They also offer a one-way rental, so for a really great trip (and workout) I'd rent in Cambridge and drop off at Brighton.