Can't find a picture, but we took our 2 year old down the San Juan last year and I got lots of compliments on my set up.
Took the old car seat (bucket style), removed the straps so they couldn't buckle in and used the latches to attach to d rings on the back tube behind me in the drivers seat. Great nap spot and pretty darn secure. It was a graco snugride 35 that has a sun shade built into it which made it even nicer.
As others mentioned, toys, sunscreen and shade and lots of stories loaded and ready to tell. Play games.
We will be taking the 5 and now 3 year old for 7 days on the San Juan this year and are looking forward to it.