This is a very fun port to visit. We didn't reserve an excursion since we were able to easily walk to anything we wanted to see. It's about a mile and a half from where we docked to the furthest thing in Key West, with lots to see along the way. We walked along Duval street to the Butterfly Conservatory and then down to Higgs Beach and the Southernmost Point. The Butterfly Conservatory was very enjoyable, gorgeous butterflies and foliage. It was a bit overpriced for what you get but I'd recommend going there. Walked around and viewed the lighthouse, Hemmingway House, many shops and talked to street artists. If you choose this port, know that many things don't open until 9 am or later and most of the fun gets going in late afternoon/evening. Our ship arrived at 7:30 am and left at 1:30 pm so we missed a lot.
This was my third time to Cozumel and it was fabulous, as always. All three times, we've visited Chankanaab on our own. It's a short, inexpensive taxi ride. The park is beautiful with lots to do, great service and rarely crowded. Snorkeling is one of the best I've done, though you'll want to get there early since it's not as good later in the day. If you choose this port, go with a cruise that gets you there early. Most of the beaches close at 5 and it's not known for much nightlife. The shops and restaurants at the port are nice to visit.