I went inland for a couple of days and while there did the Actun Tunichal Muknal. It was awesome. I loved it!
It is not for everyone though. There's a fairly easy hike in through the jungle, but it takes about 45 minutes and you need to wade through a boulder filled stream about 3 times both on your way in and again on your way out. Also, the trail can get quite slippery when wet. When you finally arrive, you have to climb down some huge moss-covered boulders and enter into about 15'of water. You then swim 15'or so to the ledge inside the mouth of the cave and your adventure really begins. You spend about 4 hours in the cave, hiking, climbing, clammering up on rocks, scrambling up and down onto ledges, squirming through one or two tighter places, wading, a little bit more swimming. It is all worth it and all the muscle pain I had after it was over. However, if you are fit and are younger, it won't be too hard a trip at all. I think being taller would have been a bit of an advantage too. A couple of times my stubby little legs just couldn't make the reaches and the guide had to assist me. As long as I followed his instructions, there was no problem at all.