Kids kayaks can be cheap and an easy introduction (like the $100 Wave). They are easy to transport and kids love playing with them. Our 5 year old is a competent lake paddler now, after a few yaers messing around on his own kayak. I started him on my lap in my kayak, them put him in the rear gear hatch when he was 2 and 3.
I would like a tandem kayak, but they are hard for just one person to load onto a car.