The best views in Thailand for sure. Rent a good motorbike, you don't really want to loose time on fixing it all the time. We made it in 5 days and wasn't long enough, people are nice even though they don't speak english they try to help. Rooms are fairly cheap, no need to prebook, just have a look beforehand where you'd like to go. Roads are fine, there are 7 elevens everywhere, mini gas stations too so no need to worry.
It takes less than an hour to get to the hill from town on foot, you can opt for a taxi too of course. To get to the peak you can take the escalator or elevator. Very crowded for sunset! I'd recommend coming earlier or after to avoid tourists. The view from the Hill is amazing, you can see whole city and around! The pagoda on the top is nice, people come to pray there.
A bit touristy but nice, mountains emerge from flat surroundings. While you're there you might be asked by local students to have a chat with them so they can practice english with you.
It's highly recommend to stop at the Visitor Information Station at 9,200 ft to receive a current weather update, safety information, and to adjust to the change in altitude. Mauna Kea is one of the only places in the world where you can drive from sea level to 14,000 feet in about 2 hours, so altitude sickness is a high possibility. At 14,000 feet, there is 40% less oxygen than at sea level, so visitors should acclimatize to the altitude before proceeding further up the mountain. Anyone in poor health should consult their physician before planning a visit to Mauna Kea.