AS Indysteel says, Hanauma Bay is great for snorkeling......

There are some great places on the North Shore and places to camp although the waves are pretty rough for swimming at the moment..... there are plenty of good hikes, whereabouts are you staying and do you have access to car ?

There's not much good mountain biking unless you like downhill over rocks You could hire a road bike from The Bike Shop and head out to Hawaii Kai area.... that's probably the safest road riding on the island. The drivers are mostly considerate but the road conditions are pretty bad....

It's high tourist season here, so unless you like touristy stuff, stay away from Waikiki......Kailua is awesome place although can be busy, great place for kayaking, paddle boarding or kite surfing.........PM if you want some better beaches to head out to or any other info.....

PS - bring light weight weather gear as it's been raining heaps, however still real hot !