Since we have visited quite a few world volcanoes and taken tours (Costa Rica, Spain, Argentina, New Zealand, etc.) we know quite a bit about volcanoes but still had swiss cheese knowledge (a lot of meat but big holes too). So we decided to take a private guided tour with a Geologist at Volcano National Park. Philip Ong used to work for the government monitoring center at VNP, has a masters degree in Geology, and is truly an expert who filled in all our gaps, was able to answer every question, and is expert enough to serve as a guide for science and technology visitors who start out with quite a bit of knowledge. In fact, another well liked guide (often mentioned on this forum) recommended Philip when he learned that we had visited many other volcanoes in the past. So, if you want to learn a lot, have unlimited questions, and want to experience time with an expert, this is the best possible choice. He brings snacks, knows his way around, can recommend good night time viewing areas, and takes you on as many hikes as you want but he is not funny and is a more thorough volcano guide. His co-worker, Arthur, is the one recommended for children, laughter, and more general volcano and nature oriented tours. They have two vehicles (one for smaller groups and another that can accommodate up to 6 people) and together they do a great job of meeting individualized needs.
We found that by having this choice of guides, we were able to go with the one who best met our needs. We liked the small personalized group as opposed to groups of 30 people we saw herded around looking glazed and we liked that we could focus on what WE were interested in as compared to a more general tour that is one size fits all. I can strongly recommend Philip and Arthur at email@example.com