It is a long walk to the Southernmost point, and not really earthshakingly amazing. The Trolley tour is good, and if the day is hot, the breezes when the trolley moves will cool you off. It drives right by the point, but the actual stop is several blocks away. May be worth the investment, coupons on the web. You can jump on and off at lots of stops. Some drivers are more entertaining than others, and even if you repeat parts of the route, you will hear new information. There are plenty of interesting places to walk around on the tour without popping for tickets for Hemmingway's and the Light House, for example. We did pop for $40 worth of tickets to see the Mel Fisher Treasure (walkable easy from the ship), too much $$$$ IMHO, but the kids loved hefting the gold. Most of the stuff in Key West is pretty pricey. If you opt out of the tour, just walk around a bit, the oldest and most interesting part of KW is right near the dock.
We took a taxi in Cozumel, quite cheap. You catch them across from the dock near the shopping center, take the escalator up and over the street, then back down again. Know where you want to go, and be insistent. We wanted to go to Dzul Ha, a good snorkel site, but the driver kept saying "no, no", wanting to take us to one of the more developed (read: admission required, kick-back received) beaches. Finally he took us, smoldering a bit. It is not as far south as the fancy places, but was great and FREE. No sand entry, but restrooms across the road, a fresh shower on the raised sea wall, sand under the "restaurant" palapas, a shack with snorkel and dive rental. We bought beer and soda in between snorkeling forays and kept our gear at one of the tables and used the chairs, no problem. Cost for four about $20 for a great time. We watched all the expensive snorkel boats cruise by and drop folks in the same water. Did I mention the snorkeling was fantastic? Spent most of the day there, very easy to hail a return cab on the road a few steps away. If snorkeling is not your intended activity, there are plenty of sandier beaches, just check out the web, Dave's site has links.
Carla