Hi Graham
Was in Hawaii in June just gone and also visited back in June 2012. There are no hotels close to Mauna Kea itself, but the area around Waikoloa is very nice. The Marriott Resort at Waikoloa is very good. From there it is about a 1.5 hour drive to the VIS on Mauna Kea. Be aware that you usually shouldn't travel to the summit of Mauna Kea without a true 4WD. The road is very steep and rough in places. There is info here
http://www.ifa.hawaii.edu/info/vis/v...he-summit.html . The VIS does run escorted tours (see here
http://www.ifa.hawaii.edu/info/vis/v...mit-tours.html ) but you must have a true 4WD to participate. They also have public star gazing nights but only at the VIS not at the summit.
Also be aware that there are protesters on the summit road who have a camp at the VIS and the road may be shut down by them.
Also be aware that access to the scopes themselves is very limited. There is a gallery in the Keck building but it is only open occasionally.
An alternative is to take a commercial tour. I have used Mauna Kea Summit Adventures and they were very good. the observing was so so as it is tailored for non astro types but the rest of the trip was great. their website is
www.maunakea.com Pickups are easy at Wailkoloa as the meeting place they use is at the shops over the road.
One other thing with the commercial tour is you are there for sunset on the summit (be prepared to be very cold!!) but you do get to see some of the domes start to open for the night! I wouldn't be there after sunset and attempt to drive down myself, low oxygen, dark and a bad road are a nasty combination!!
The Keck observatory has a base at Waimea north of Waikoloa, that is quite interesting. Might be worth checking out.
There are telescopes on top of Haleakala on Maui but no public access to that part of the summit, but you can see them from the visitor centre about 1km away.
Other things to do that are fun are snorkelling, best place for beginners on Oahu is Hanamua Bay, visiting the Volcanoes on the Big Island is fun, did a day trip with Hawaii Forest and Trail and they were very good (see
http://www.hawaii-forest.com/) and they also do Summit tours. Helicopters are great on Hawaii, Blue Hawaii Helis run flights from the little strip at Waikoloa, not cheap but one of the best things I did.
On Maui (if going which I recommend) try to be there on a Friday and check out one of the Friday Night Street parties. Details are here
http://mauifridays.com/ we did the one at Wailuku and it was a lot of fun.
Good luck with the trip!!
Malcolm