Lots of good diving along Ali'i Drive where the Royal Kona is located. If your looking to shore dive, you couldn't have situated yourself in a better place. The sites are all pretty easy to get too, however a guide will make it much more enjoyable both getting in at the right spot as well a specific areas to check-out. The limiting factor here is during winter and spring months the surf can prevent shore diving at many of the sites. I moved here for the shore diving!
In this case, best to book a couple days of diving to include a Manta, if you really want to maximize your time here, do a morning 2-tank and then an 2-tank manta in the same day. Not all operators do this, check around. If you have dove recently i would recommend the Manta dive first, then do a day dive the next morning, again this will maximize your time to do other things.
have fun while your here....and yes just rent a car, I just had a friend rent a compact for $98 for 5 days.
brian