Hi MXS,
Two days is pretty quick to see the Big Island but you're making the right call by driving to the volcano. It's a pretty easy self drive tour. Be sure to walk through the thurston lava tube. Very cool.
If you're staying in Kona another activity I'd suggest is to go see the Manta Rays at night. The
Sheraton Keauhou has the mantas right off shore. You can grab dinner or cocktails at the hotel and walk out to the edge of the ocean and see the mantas. The lights from the hotel and diving boats attract plankton which brings in the big mantas. It's a fun thing to do at night after doing day time activities. If you're a little more adventurous you can reserve a spot on one of the manta tour boats. It's completely safe and you can either dive or snorkel. I opted for the snorkel and it was one of my favorite experiences.
Have a great time!
Jim Bendt - Owner - Travel Beyond