How about one of the Hotels downtown. Many of the Hotels have nice large lobbies with nice flower arangements and beautiful decor. Most Hotels won't mind if you take pictures without charging a fee, and then if the weather turns out to be nice you could head down to the Lake front for additional pictures...Just some suggestions from Badger State Limousine Service and Simple Ministries Good luck and congrats.
I was going to suggest heading downtown to a hotel, too. The Pfister is super-pretty & classy. Or, if you're into a more modern scene, the art museum is gorgeous & would have lots of natural light from all of the windows inside (my guess is that you could sneak into the lobby free?). Otherwise, the domes might give you a good floral-y background, but my guess is you'd have to pay admission. The Wicked Hop or another one of the historic bars in the 3rd ward might also be a cool spot for some photo ops. Best Wishes!
If there is snow, I think it would be pretty by the lake, particularly near trees. Southshore Park (which is where we are getting married) has the best of both worlds- trees and the water. And it's definitely free!