Mall at Milenia is gorgeous! It's huge, it's brand spankin' new, and it's about as elegant as a mall can be. That said, it's also mostly stores you'll find near home, but some of the restaurants are definitely worth the drive over. Beautiful place, easily worth a few hours time.
Premium Outlets is very nice, too. Lots of designer brands in a bright, cheerful atmosphere. There's a food court, and if you can get ahold of their coupon book there are some very good deals to be had.
Florida Mall is another option, not as upscale and trendy but with enough to recommend it as a possibility.
Belz is not my cup of tea, though it is hugely popular.
Festival Bay is another choice, though I'm not sure it's filled yet. When (if) it is, it will be a very nice shopping venue, with some unusual stores that you won't find just anywhere. Maybe worth a stop in as you're out and about. Ron Jons and a huge outdoor center are their anchors so far.
Enjoy! There are lots of great shopping options in the Orlando area!