The hike up to Diamond Head is close by.
Ala Moana mall is nice to walk round at, with a few eating options. If you like dim sum Jade Dynasty has a good one, and Genki sushi has the conveyor belt sushi but I like it.
My favorite sushi is Kenko Ya, but that might be too far away.
My favorite snorkeling is Hanauma Bay, there is probably a bus that goes out there, and there is a small fee to get in.
People say Perl Harbor is cool, but I've never been. There are buses that go out that way.
Remember to apply the sun screen liberally!