I liked the volcanic wasteland at Volcanoes National Park, and we really liked Kaumana Caves Lava tube, depending on your claustrophobia (some sort of head protection might be good for the low ceilings).
Snorkeling at Captain cook, and at on a couple of beaches near Hilo (Richardson and Carlsmith). Also Sushi in Hilo at Ocean Sushi.
Sam recommends getting the same guide book that we (read: she) was so good at using. Hawaii Revealed
Zan, I'll bring you that guidebook this weekend if you want. Also, our neoprene gloves to see if they fit.
Yep, that guidebook is really where we got all our ideas from - everything from where to eat, to where to snorkel (though that depends on the season), to what trails to run in Volcano. I really loved their suggestions. I also bought the app and thought it was totally worth it, as you could search for stuff nearby and use their offline map.
Also I will second the lava tube that Ross mentioned. Wear sneakers and bring good headlamps and you can really explore! And Ocean sushi was also great because it was reasonable priced and super yummy. Also Sushi Rock in Hawi had some real interesting sushi combination and was a good post run (and rope climb) adventure. Captain Cook had the best snorkeling, though it is a pain to get to, but there were even dolphins swimming with the snorklers while we were there! Two-Step is also good (parking limited) and much easier to access. And I can share with you the airbnbs that we stayed in if you're interested. I would highly recommend them all for different reasons. The place we stayed in Volcano was by far the most beautiful! Ok, I would love to just keep going and going, so let me know if you want more specifc information!
We rented from Hertz and both times they were a bit of a pain in the butt - really long wait time for the cars. But reliable cars, so that was nice for all the driving we were doing.