Fun things to do on Samui with 3 year old

Finally, TT is back up, and just in time for our upcoming trip! We'll be spending 3 weeks on Ko Samui in March/April with our 3-year old son and I'm researching fun things to do for him while we're there. This will be our 7th trip to Thailand and we've been to Samui last year as well, but he was only (just) 2 then and we didn't do much besides go to the beach. I've already found a waterpark, which seems like something he'd enjoy for 2 hours or so. Anything else that could be fun for him to do (besides the beach)?
We're staying in an appartment (Samui Emerald Condotel) close to Bo Phut/Plai Laem, so I guess we'll probably be spending a lot of time on Choengmon beach again, but we'll want to see a bit more of the island this time. Nothing too strenuous though, since I'll be 6,5 months a long with nr. 2 :-). Thanks!