I really liked Wasabi over in Kailua Kona for sushi, it's just off Ali'i Drive. The service a few years ago was really slow but the food was so good, we went back last year and it was one of the best meals we had! Also in that area was Longboard pizza which was really good.
Two Steps is the place to snorkel and has lots of sea turtles on one side. It's near Puuhonua but not the national park. It's next to that.
If you will be there on a Wednesday or Saturday you should not miss the Hilo Farmers Market.
Have fun!