The sea horse farm is cool, the tuna farm for KONA BLUE is also interesting as well as a don't miss for the volcano. The observatory trip is interesting too. A trip to the coffee plantations is also of interest. If you are trying to stretch your travel dollars many of these things can be done for free or a small cost.
I dive everyday for three months at a time off the West side of the big island from the airport to Southpoint. It doesn't look like much chance for beach diving in this area. Honu and Jack's are the two operators I'd pick, bigger boats. Ask for a rate on several dives. Since airfare has gone sky high, tourism has fallen on the island. People are more willing to make a deal. If you do the manta dive, try to book with a shop that will give you a rain check if no mantas show. We got our first blowout in 8 months a week ago. We tried 2 nights that week. Choose a shop that goes to the Sheridan and the airport sites. Ask about the manta reports for the last few days for each site.