If you haven't booked your boat dives yet and are on a budget, Kona has great shore diving.
When my wife and I are there, we use Sandwich Isle divers for our go to shop. The people there are great. We head there in the morning and pick up four tanks, the owner grabs a tourist map and marks out a couple good dive sights, gives us all the details on entry, parking, etc and off we go. We head back to the resort (Kona Coast), have a little lunch and a rest up a bit. Then head back to the shop swap out the bottles for another four and repeat.... The only boat diving we do is the Manta night dive and the Pelagic magic dive. The way the island is formed, it gets deep just off shore, so pretty much any diving you will get from a boat, you can find from the beach.
For a day trip, it's fun to rent kayak's and paddle out to the captain Cooks monument. We packed our gear out there and the diving was a really nice wall, just off the monument.
There are a number of good spots with decent beaches and the Place of Refuge seems to always have turtles.
As far as fun things to do, plan on doing some driving. Most of the tourist type activities are pretty spread out. One spot we enjoy is in downtown. There is a women there that has a shop that makes the most incredible clay orchids, we always stop to see her display's.