Oh boy, brainstorming......
Time: Like Kelzie said, I think three days would be ideal. If we shoot for October, Columbus Day weekend would be a good goal. A lot of folks would already have three or four day weekends for that, so they wouldn't need to take vacation time, and it's not such a popular holiday that places would be booked solid, nor would prices be outrageously jacked up.
Places: Most of the cities that have been suggested thus far are good touristy sorts of spots. Vegas, D.C., New York, Boston, Philly. All have historical stuff, barring Vegas, really; and all also have plenty of other attractions as well.
Charges: americanwoman mentioned that she's a travel agent, so she could probably help us with perhaps a group discount with a hotel. Also, if we used the hotel's conference rooms, rather than renting out a whole convention center, they might have some sort of package deal with that, maybe even with some catering thrown in. What about airline discounts, if we were all able to book our flights together? I know we wouldn't all be flying out of the same place, but it might be worth looking into.
Entertainment: A number of our members play instruments, perhaps we could have them play a bit, maybe even make a contest out of it....
Another angle on this that I mentioned to Kelzie would be making sure that we'd have some sort of childcare available. I'm sure plenty of folks would love to bring their spouses and children, but toddlers would be bouncing off the walls and older children would get a bit bored, I imagine. And some of us wouldn't even have the option of leaving the kids home...I wouldn't have anyone that could watch the baby for me (if hubby isn't able to come, it'd be because he'd have to work, so I couldn't just leave the baby with him), nor would I even feel comfortable leaving my baby that soon!! Just another thing to keep in mind.