The ferry that leaves from Miami's Key Biscayne called the Key West Express Is a $98 round trip and you don't have to leave the same day.
However, I've never had a problem parking in Key West. People who do are usually tightwads!!!
You will always find parking if you are willing to pay for it. John Pennekamp Coral Reef State Park is in Key Largo. If you go, it is much more worth it paying s#!t I don't know what it costs now, but it's worth paying the premium to take the boat out to the protected coral reef to go snorkeling. It's absolutely magnificant. Key Largo also has the Jule's Undersea Lodge, which has a lagoon that is safer then that crappy beach at Pennekamp's. However the main attraction you have to be a certified S.C.U.B.A. diver for which is the undersea lodge itself 30ft. below.
The drive down the Overseas Highway aka U.S. 1, is great in and of itself especially the Seven Mile Bridge and seeing what is left of Flagler's old East Coast Railroad that was destroyed by the Hurricane of 1935. Make sure, that before you leave Miami-Dade Co., you fuel up. Florida City is the last stop in Miami-Dade before the long drive into Monroe Co. and once you get into Monroe, gas goes WAY WAY UP!!!!!! One cheap stop and one only is Tom Thumb's in Tavernier. It's the cheapest gas station in the Keys. So worse case, fuel there.
However, if you opt to take a ferry, you can always rent a golf cart or a moped which is an extremely efficient mode of transportation in Bone Key.