Cozumel
if you like ziplining - you will love XPLOR park at Playa del Carmen (Cozumel). It is on the mainland. When you book through the cruise, the ferry docks where
DCL docks, so its a short walk, then the crossing to the mainland, a short walk through town, a short bus ride, no lines to get into the park.
Its an expensive excursion but we have had a blast both times we did it.
There are two zipline courses - do them back to back so you don't have to wait to gear up a second time
There is jeep driving course(s) - both on land and in tunnels underground
There is the underground river - a short course and a longer course - if you have time do the longer course - the exit is Awesome!!
The underground river also has rafts - for one and two people - I'd do the "swimming, walking, bobbing along" (above) before doing the rafts
Lunch Buffet - OK, not horrible, not great
We wore our swimsuits under our clothes on the trip over so after we got our locker all we had to do was peel off the outside layer toss it in the locker and off we went. Remember First up is ziplining so for the ladies, wear shorts and a mesh shirt, guys, also wear the mesh top. There are a couple of waterslides in the middle of the zipline courses.
You possibly could arrange this excursion on your own, but the transportation issues (ferry, taxi) make booking through DCL the better option in my opinion - unless DCL is staying LATE in Coz (they do this sometimes)
Jamaica
And here is a second for the "Falls and Bobsled" excursion in Jamaica!!! Walking up Dunn's River Falls was a blast and is right up there as one of the coolest things that we have done. Bring $40 to buy the DVD they make of your climb (I think thats what we paid - might have only been $20). As a note - you WILL get wet climbing the falls - dress appropriately. I didn't bring a dry shirt to change into (no bottoms either). I would have preferred climbing (walking really) up wearing a mesh shirt along with my swim suit, then having the dry shirt to change into.
The bobsleds are fun, but it is to California Screamin' what Humunga Kowabunga is to the AquaDuck - fun!!

but its not

a thrill ride. That being said, I would consider skipping the lunch that is served up there and purchase additional rides on the bobsled. Then grab a quick plate and eat on the chairlift ride back down the mountain. Due to the number of people on this excursion, half go to the Dunn's Falls first and the other half do the Bobsleds / lunch first.