Some suggestions:
Lake Crescent Lodge (Washington State)
Estes Park, Colorado (there's a couple of large, nice hotels up there, it will have to be summer)
Gardiner, Montana (ditto the summer condition)
I'd suggest Moab, Utah, but I don't know if there's any facilities that would work there, they're mostly "rock climber" facilities.
I'd suggest Apgar Village in Glacier National Park, but I don't think the chance of finding space there in the summer is very good.
I could be a wise guy and suggest that we rent the Corn Palace, but that is so far from anywhere, even Lisa
Hmm. Maybe Cedar Point, OH, and we could do some exercises in altered 'G' forces on 'Millenium Force'... Naah. Stick to the ones above, sorry.
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Edited to add:
There's also Coulter Bay Village, etc, the areas in the Tetons, but they are for a more, um, expensive class of people than I'm used to hanging out with.
Seriously, the 'Yellowstone Inn' in Gardiner has a big "barn" structure that will keep the rain and the chill off for groups, and lots and lots of rooms.
How many people are we talking about here?