My family and i stayed at the Fontainebleu when it was a Hilton hotel, before all the renovations. It was fabulous! We LOVED it! Great hotel, great pool, great service. They also have a great pool bar that has really good "virgin daiquiris" for the kids and good food.
We've also stayed at Eden Roc, which was fabulous as well but I liked the Fontainebleu better. We've stayed at I think it was a Courtyard and a Days Inn, which were nice (as long as they have somewhere to sit by the pool, my family's happy as we spend most of the day by the pool or walking along Collins and go out in the late afternoons/night). The one place we stayed that I was not very impressed with was down closer to South Beach. I can't remember the name of it, but we really did not fit in there. We were much happier further up Collins.
Have you been there before? You definitely have to go to Jerry's Deli for a meal. We usually end up eating several meals there and go every night for dessert. They have AMAZING desserts and a HUGE menu.
Another thing you might want to try to do, if you're interested in this kind of thing and have the time, is Jungle Island, which is a zoo-esque place. The VIP experience was one of the coolest things I have done in my life (I'm very interested in animals, though, and am going to school to hopefully eventually work in a zoo). It's a (private, with only you and your family)behind the scenes tour with encounters with all kinds of animals. We got to play with penguins, feed orangutans, and the rest of the group (I have an extreme phobia of snakes and anything snake-like, which the tour guide and keepers were fascinated with, so I left for that portion) held a HUGE snake. The best part, though, was the lemurs (which you can do alone if you don't want to do the whole VIP experience). We went into a room, sat on the floor, and the let probably about 15 lemurs out of cages. They were climbing all over us, we were holding them and feeding them. It was very cool.