Yeah, unless it's changed, the rental car contracts say you can't take the cars on the road you need to take to get there. We drove it (didn't see it until it was too late) and it was kinda rough, but not that bad. We drove up to the observatory on top (it was late september and "cold as shit"), but that was probably the 40s or 50s. (There's nothing there (uless it's changed) except an observatory and rocks - lots of rocks......
We checked into the skiing, and the way it worked (this was in the early 90s) was that you could get packages where they would drive you up in a 4-wheeler, and rent you skis. They said they only get enough snow to do it every 3-4 years, but if you're on the island of Hawaii, it would be worth asking about (if it's winter...). Having said that, winter isn't the best time of year weather-wise to visit Hawaii if you want to hit the beaches (can be cloudy for a couple of weeks at a time.)
