Do this excursion! You can pruchase tickets at their website hiddenworlds.com. What an awesome time. Skycycle, zipline, snorkeling cenotes, and rappeling. A taxi is rather expensive $110.00 round trip. But with the money you save booking on your own (not thru carnival) the price would be about the same as their excusions and you can stay all day. You can do the activities as many times as you like. At the end of the day they show you a cd of pictures that were taken of your group during the day. We had over 150 pictures (didn't even notice photographer) and the cd was only $25. My family will remember this day forever.
Also another option is to take the public bus. Which is what we did. We took a taxi to the main hwy, walked across the street (have to be on the side of the road for the direction you want to go) bus pulled right over and picked us up, dropped us off right in front of hidden worlds, and when we were done for the day we walked right across the street, bus picked us up, let us out at dirt road to Calica port (you'll see what I mean when you get there), then another taxi came by, we flagged him down and he took us directly to the ship. Taxi $10 - total - Bus $3.50 per person each way. Was very safe. Had no problems as all