Bang Krachao 47km
Wow. What a fascinating place, and only a five minute boat ride from central Bangkok.
Tiny paved tracks, through a mangrove Swamp, that people actually live in, so houses, shops (stalls), temples, canals, washing being hung out, dogs, kids, but so tranquil. And then you can stop and take photos of the city across the river.
Thanks Derek Williams for the recommendation. Lovely, interesting riding. And the ferry was NOT big.