Any tips for visiting Cairns?

Going to be visiting Cairns soon, to meet up with some family (they live in Papua New Guinea, and we picked it as a sort of middle ground).

I’ve never traveled internationally before, and rarely leave the Midwest USA, so I’m pretty excited.

Any recommendations for things to do there? Ideally stuff that a kid could come along with, I want to spend as much time with the nephews as possible.

Also, anything in particular I should avoid doing to not be rude?

Edit: Wow, that's a lot of advice. Thanks guys! I've taken note of the big suggestions, as well as the dozen or so people telling me not to get eaten by a crocodile. No way I can reply to all of you, but I appreciate it!