Don't know much about Miami, but my parents and sister used to live in Key Largo and I have spent a bunch of time there. On the way down, take the long way to the key by taking Card Sound Rd. Turn left just south of Florida City. Stop at Alabama Jacks on Card Sound Road for lunch and to check out the live entertainment. Back in the 90's a Wesley Snipes film called Jump Zone was partially filmed in Alabama Jacks. Once in Key Largo, check out the Caribbean Club (bay side approximately mm 105) The Humphrey Bogart movie Key Largo was partially filmed there. A very eclectic group could be there...locals, bikers, tourists, etc. I have always had a blast there. Pennycamp Park is a good spot to check out. Hobo's is a good spot to check out for dinner, and the Hideout for breakfast/lunch. If you take US 1 all the way down (or back up), check out Gilbert's on the north side of the Jewfish Creek bridge. I really recommend using Card Sound Road at least one way on your trip to Key Largo. It's a neat drive and it's got Alabama Jacks!
None of these places are "nightlife" but chill, local keys places with good food, music, and waterfront bars.