Hi Graham
I was in Hawaii back in June. As already mentioned the Big Island is the spot for astro. Oahu (where Honolulu is) is really very touristy, though there are some nice spots. Fares to get to the Big Isl;and are not prohibitive, so try to get there.
I did a trip to the summit on Mauna Kea with Mauna Kea Summit Adventures (
http://www.maunakea.com/ ) They take you up in the evening have a meal and get acclimatised at the Visitor Centre and up to the summit for a look at the scopes. Watching the sunset up there is brilliant. We then had a few hours stargazing with a couple of C11's down at about 9000ft. The sky is amazing and the views in the scope are awesome but having to share 2 scopes with 30 others was a bit tiring. Luckily one person had brought along a little 100mm refractor so we used that as well.
I am not sure about tours inside the actual observatories but I amsure that can be arranged.
With the altitude, at the summit you certainly notice it, especially if you sit down and then stand up suddenly, but if you are in at least reasonable health and fitness should never be an issue. I went up twice and second time acclimitised for 45 minutes and was easy. First time I rushed it a bit and felt a little bit of lightheadedness, but nothing serious. We had peaple on the Transit tour in their 70s and 80s and they all handled it well.
Also although it is not cheap, a helicopter flight with Blue Hawaii helicopters (
http://www.bluehawaiian.com/) I did the Big Island Spectacular tour which is about 2 hours and worth every cent.
The helis are the best way to see lava and volcanoes, unless you are VERY lucky the ground based tours will not allow you to get very close at all. Nevertheless the tour I did with Hawaiin Forest and Trail (
http://www.hawaii-forest.com/) was very enjoyable. They also do MK summit tours.
I can highly recommend the Marriott Beach resort at Waikoloa, very comfortable, lots of activities, much nicer setting than Kona or Hilo, ancient Hawaiian ruin just down the road, lots (and I mean lots!) of lava.
One interesting thing you will find, especially if you head up the saddle road (goes across the island between MK and Mauna Loa) is how much some of the country looks like Australia!
Thats all I can think of off the top of my head.
Malcolm