If it were up to me, I would do it backwards if possible. I did the "bag drag" both ways last year and enjoyed, but it would have been so much better had I flown out of Cozumel on the way back after a long week of diving!
In other words, visit Tulum then bag drag to Coz and fly back home once the invasion is done directly from Coz.
Also timing your flight out of CUN with the bag drag is always sketchy, I had a very late flight out so I had plenty of time to get to the airport.
I can't rearrange it because of my work schedule. I actually ended up having to take an extra day off. I was scheduled to work on Sat. the 30th. Otherwise I'm with you and that would be a great way to do it. All my extra days off fall at the end of the invasion though. I don't return to work until Thursday the 12th. And I'm supposed to dive the Aquarium on Wed. the 11th but I'm not sure I can bring myself to go from the warm waters of Coz to 76 degrees in the aquarium. Might use that day to bail the dog out of the kennel and do laundry!
