It looks like out of all the people responding to this thread, that Nashville is the obvious choice if you want the most people to show up. It's pretty central also no matter which direction.
I prefer Tampa, (duh, its close) but honestly I don't think most of you could handle the humidity down here.... and I would also guess that most of you aren't 'beach folks', but I could be mistaken.
There is a ton to do down here that isn't beach related, but to me that's the best thing about the area.
As for cruises, Hurricane season in Oct is the BEST time to go IMO, the prices are cheaper and historically there are very few, if any, storms that month. You can def have it "In Season" when that isnt a concern, but it will be a bit more expensive. We took one on a 5 day to the Bahamas and it was very afforadable. Once you are onboard you only pay for drinks and upgrades (steak dinner, etc)
It was cheaper than a lot of vacations I have had. Think about room charges and food for one day only on any given trip, and a cruse is usually cheaper.