Just got off the Spirit Sunday... The itinerary is not a reason to book this cruise - it's rather boring. We didn't book any shore excursions at all; they are almost always so overpriced and not worth it. In Port Canaveral some folks went to Disney or Universal, and complained that they felt like they only got to glance at the parks before having to leave. The Space Center is visible from the ship; closer, but we've been there so we skipped that as well. (Heck, my kids spent a week there at Space Camp). it was also too cold that day to bother with a beach.
Miami was also chilly; we opted to have a relative from the area pick us up and drive us to South Beach.
Great Stirrup Cay is a joke. Beautiful island with a small beach that will comfortably accommodate several hundred people on a nice day. Put the whole ship there and... It was a zoo - wall to wall people, hardly any room to walk or move on the beach. If you want a lounge chair or hammock be on the first tender over. Otherwise you'll just get a little chair; they're packed in like sardines. If you want to snorkel bring your own equipment. $29 to rent theirs is ridiculous! Lifejackets for snorkeling are free and required. The food is about the same as that on the ship. Terrrible.
Nassau is nice, for the few hours you are there. You can get off the ship at 7am, but nothing really opens up until 9 anyway. Lots of shops, stores, strawmarket, and so on. Typcial tourist trap. You're not really there long enough to venture far.
Happy to answer any specific questions!