I spent a good bit of time thinking about the hitch. My favorite part is using a short piece of automotive heater hose to wrap around the bike seat post. It flexes a little, absorbs small shocks, and attaches and removes quickly with no tools. I like it.
The trailer has a wooden frame with low-speed low-pressure wheels, slightly smaller than wheelbarrow wheels. They roll nicely and absorb minor shocks.
It's all connected by a long piece of black iron pipe, bent to the contours of the rear bike wheel and the bow of the kayak. And the pipe swivels in the frame so everything moves.
Here's the instructable: http://www.instructables.com/id/Bike-Kayak-Trailer/
and the 16-second video: http://youtu.be/GovHLDrF2eU
With a little paint, and some red blinkies for those full-moon cruises, it'll be all set. I just hope she doesn't try to back it down the boat ramp...