The top of the largest Maui volcano (1 of 2) is Haleakaia the House of the Sun”. It is part of the National Parks system so it will cost a few bucks to get in.

At the top this mountain, which is about 10,000 feet above sea level and 24,000 feet tall if you drained the Pacific ocean (making it the largest mountain in the world, if you drained the ocean), is a Visitor’s Center, US Air Force optical telescope (they claim they use it to peep on space junk but I think, and this will get me into trouble, that it was built for President Reagan’s Space Wars to track Chinese missiles as they cruise to the US) and University of Hawaii telescopes. You can visit the first and the later.