OK, so you don't have a choice. There's only one road thru the keys and that's US Hwy 1 (see map). New bridges parallel the old ones, with great views. On Key Largo, you can snorkel above the offshore coral reef at John Pennecamp State Park. Saltwater fishermen can stop at the Worldwide Sportsman in Islamorada, not to mention being able to hire any number of charter boats. And then there's Key West with the fabulous sunset at Mallory Square. Don't miss Hwy 1A, on the south coast of Key West.
Maps.com has your Florida Maps!
If you've never been, here's a preview from Robin Collins' trip.