Do be aware that if you just walk in off the street at say 8.00 in the evening and ask for a rate for the night there is a good chance the hotel will quote you "rack rate" as they know it's a hassle at that time of night to search for a top rate at an empty hotel. Also if you do happen to hit a busy weekend, you might find you waste a lot of time.
As JJ says I would try to plan at least one day ahead of changes of location. I.E. if you plan on spending 4 days in Orlando then moving to Tampa I'd find my way to an Internet Cafe for an hour on the third day to check up room rates in Tampa. It may cost you $5-10 in the cafe, but it'll probably save you $30 a night a room fees and a lot of time wandering about. If all you're looking for is clean, cheap rooms I would suggest checking out
http://www.hotwire.com . They do also often have a feature for booking rooms up to 6.00 in the evening of the check in (but I wouldn't leave it that tight). For spots that have high volume of hotels (like South Beach Miami) I've had some stunning deals at short notice. DO be aware though that some areas (like Miami) many of the hotels have a habit of charging a "resort facilities fee" on top of the regular room rate as well as charging for parking. IMHO this is a bit of a confidence trick, because it's not an optional extra like "if I don't use the pool/beach can I not pay the fees?" but the resorts use it as a ploy to keep down their "headline" room rate, while still sneaking a few extra $$s out of you. This can vary in size ( most I had was $12 a night for resort fee and $8 a night parking/$12 if valet parked) , so do ask when you check direct with the hotels. Unfortunately on hotwire you can't see what the "resort fees" are going to be (if at all). I still managed to get a beautiful $200+ a night 4* hotel on South beach for $60 (+plus $8 "resort fee" and $8 to park) on hotwire so I was more than happy with the deal.
Carhire companies will charge you a fee (can be quite high) to drop off at a different airport. Airlines usually will add cost to fly into/out of different airports. As most places are within a 4 hour drive that you're likely to fly from (Orlando/Tampa/Miami) it's going to be a lot cheaper and easier to use the same airport into and out of the USA if you're sticking around central/southern Florida.
In your situation, I'd see what airport is the cheapest to fly into (it can make up to GBP 100 a ticket difference) and plan my trip around that. Miami is usually the cheapest airport to use. You could easily fly into Miami, book a cheak motel on hotwire for the Miami airport area for one night, drive to Naples the following day (takes about 3 hours I think, but
http://www.mapquest.com is great for driving directions and journey times. Just find a hotel's zipcode and address for the start and end of the journey and it will give you a good estimate of time/distance ), spend a few days there, drive up to Tampa area (again I think about 3 hours) spend some time there, across to Orlando (2 hours drive) for a while , I'd probably opt for St Augustine next for something slightly different (near Jacksonville and about 3 hours from Orlando), then down the coast to Daytona (2 hours). From Daytona to Miami is probably about a 5-6 hour drive , you could break the journey at somewhere like Vero Beach (Disney have a great beachside hotel here, I'd choose Vero over Daytona in an instance, but someone more interested in motorsports may prefer to stop in Daytona) or just get down to South Beach and finish your journey here. To be honest, you don't really need a car if you're spending your last 3-4 days in this area you can walk to some great shopping,restaurants and clubs from anywhere in South Beach. If you pick a hotel with beach access and a pool you'll have plenty of daytime relaxing if you want. You can just then take a taxi to the airport on your departure day and it saves you the $cost and hassle of parking a car that you don't really need/use. Obviously if you wanted to do some excursions/exploring of Miami, then the cost/hassle of parking may be justifiable.
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