Turning 6 years old in Curaçao - what to do?!

My son is turning 6 years old in Curacao on our upcoming cruise (lucky guy!). Husband and I would like to do something special and memorable for him. We will also have our 9 year old son with us.

We plan to snorkel in the morning, then we will have plenty of time in the afternoon and evening, as we don't depart until 10 p.m. The dolphin swim is out, as it has been reserved entirely by our cruise ship, which puts it out of our price range (hard to pay what they want, when we already saw the regular prices on the dolphin web site) :-)

Thoughts on what to do?

Thanks!