Flying over to the nearest Bahamas is a great outing and pretty straightforward (just set heading 090 and sit back and wait
) There's info on the formalities here:
http://www.bahamas.com/islands/exper.../index.html?13 But, if you fancy that -- you'll need to make sure you choose a rental outfit that'll let you (ie has sufficient insurance), has an aircraft with a US Customs sticker on it, and can lend you lifejackets (& raft).
Also check what maximum rate you'll get fuel purchases refunded at, the cost varies a lot around Florida so if you rent from someplace where it's much less than $2/gal then you'll be spending quite a bit extra everytime you take on fuel. You can check fuel rates on
http://www.airnav.com/
Another fuel tip, if you're going to Key West, be sure to land at Marathon and refuel at the self-service fuel pump, south end of the apron, as it's virtually double the price elsewhere on the keys!
Check the rental contract for all sorts of other hidden costs, eg what do they do if you break down someplace? Authorise you to pay for immediate local maintenance attention, or insist on travelling out to fix it themselves, whilst you're stranded (& parked) at your expense? And check the insurance excess, if it's more than a thousand dollars that's not good.
And bear in mind if you fancy landing at Miami/MIA, it's about the only airport in Florida with a landing fee ($18), and if you want to stop off at the FBO, you'll need to pay a handling fee or buy some expensive fuel. Aswell as spend half an hour in a holding pattern whilst they try to slot you in...