I have gone kayaking twice on the Charles River using the Charles River Canoe and Kayak venues. This past weekend I went for the Fourth of July and was fantastic! For $10-15/person you could go out for an hour on the Charles! Depending on your level of experience of the heat during the day, that may be all you need. The kayaks were in good shape and the people on our trip took out the single and double kayaks. I highly recommend going to the drop off location in Kendall - this brings you directly out into the Charles in front of the Esplanade with AMAZING views of Boston along Storrow Drive. However for first timers or younger kayakers I would be ware that there are more boats, particularly sailboats, that you have to be wary of and mind the direction you are going as it is a little bit windier. If you want to go for a more quiet kayak ride, the drop off location in Allston is much quieter and narrower and not as much traffic.