If it's just the two of you I would recommend using
http://www.hotwire.com for the rooms. The best deals are usually close to arrival times, but it's worth checking on a regular basis as every now and then you get a real bargain pops up. It is a bit of a gamble using them because you don't know the name of the hotel until you actually (finally) commit yourself to the booking, but I've used them many times and have yet to have a bad hotel. I've used them all over the US, I got a great deal in Miami last Xmas at the Richmond hotel ( great place to stay) and also in the Keys, both rooms were less than half their normal "rack rate" . For example, I just put up the end of Nov and the best deal they offered a 3 star hotel for $42 a night.
In Miami I'd suggest the South Beach area, there are loads of great bars, restaurants and shops in the area and it's a lot of fun wandering around looking at the Art Deco buildings. There are a number of art galleries and museums in the area as well.
Here's a link to an attraction in Miami I've always fancied visiting
http://www.venetianpool.com/
Both the Keys and South beach do have "parking issues" so be aware that you'll probably have to pay for parking as very few of the hotels have space for cars outside the hotels. There are a number of car parks in both areas, or you could choose to "valet park " the cost is usually about $16 per day to valet ($8 per day is usual IF the hotel has "self park" available). One more "issue" is that many of the hotels on South Beach have a "facility" charge for stuff like the pool and beach access, The rate you're quoted may nopt be the final rate you pay, particularly if you have a car, the "facility" charge was about $8 per day I think for the hotel I got charged it at, not all hotels have this fee, it's a bit hit and miss I'm afraid. With Hotwire, there is the possibility that you'd get a room that only has one (queen/double) sized bed. If it's just two people that isn't going to be a problem (hopefully

) , if you want to reduce the risk of this occuring it's best to avoid the hotels that are marked as "boutique" this is denoted by an Icon of a large capital B in a box. These boutique hotels can be great fun for a couple as they are often the more unique or historical buildings. As I said, for a couple they can be a great option, but could cause problems if you're a family of 4 :O .