The helicopter/dogsled tours are fantastic. One tour specifically flies and lands on Mendenhall Glacier, but there are other options as well... Temsco, ERA and Coastal Helicopters all run these tours in the summers in Juneau. Temsco also operates a dog-camp near Skagway. I don't think it really matters which glacier you land on. They all look about the same from above, it's just that Mendenhall is the famous one because you can drive very close to the face of the glacier.
I mention Skagway because if your touring schedule has you doing something else in Juneau, there's another option. Note that these tours are cancelled fairly often due to weather... probably 4 or 5% of flights are cancelled. Be sure to have a backup plan in mind. Also, I always suggest folks book the earliest flight in the day, so that if it's foggy in the morning you'll have more of a chance to re-schedule later in the day after the fog burns off.
Booking directly with the vendor can save some fairly big bucks. Coastal Helicopters has their dogsledding excursion priced at $429 pp for 2008. When comparing, look at the flight time, time on the ice, time with the dogs, etc. The ships promote them heavily because of the over $100 per person commission they earn.
Yes, it's well worth the expense... in fact, I just booked 3 seats on Coastal in August for myself and 2 friends coming to visit.
Happy Alaska Travels!
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