I think Florean's would be a great idea!
My other favorites:
-Gringott's Coaster...everyone seems to adore this idea! Best of all, it would fit in with the DH movies/books.
-Weasley's Wizarding Wheezes...I love Fred and George, so it goes without saying that I'd hands down want this. Move all the pygmy puffs there (because that's where they actually belong) and sell things like "love potions" (soda or something in pretty bottles).
-Flourish and Blotts...this is what I want the most. They could sell The Life and Lies of Albus Dumbledore, school books, Hogwarts: A History, the Daily Prophet, Quidditch Through the Ages, The Quibbler...I think it would be really cool.
-Madam Puddifoots
-Borgin and Burke's...they could have a little alleyway that leads to Knockturn Alley and have a bunch of storefronts, and then B&B with prop replicas from the movie.
I'll try and come up with some other ones...I'm currently re-reading the books, so maybe something else'll come to mind.