I agree, Hanauma Bay is awesome for snorkeling, but like the previous poster said, get there preferably when they open, as they close the parking lot when it fills up. If you're interested in kayaking, Kailua Beach is a great place to do that. Hikes I would recommend (not really volcanos) are Makapu'u Lighthouse (just down the road from Hanauma Bay) with awesome views, or Maunawili Trail off of Pali Highway.