That Nonthaburi pier is a very dirty filthy place with loads of rude people. I wouldn't go there but up to you.
The cremationplace of the ex-king is something you won't see anywhere else, it's really beautifull. (bring passport if going there, it's free entrance and also open at night with nice lights).
Can go there by boat but have to walk all around it (through overflowing toilet crap) and after it you walk back all around it again to the boats.
I would bring them to the river dinnercruises after that. Great way to eat and see BKK in a relaxed way. Forget about chinatown but up to you of course.
For exploring, Chatuchak market or another one ( in the weekend) or the chatuchak plantmarket on wednesday/thursday. Plus ortorkor market for some fruit/food.