The Iberostar you are staying at has a great many things to do (esp the pool/wavepool etc.), so that should be fun!
+1 for Xel-Ha. My husband and I go every time we're in the Mayan, we love it so much. Especially the river (husband floats down on a tube while I snorkel in the water beside). Last time, we took our 4YO son who was nervous at the thought of swimming in such deep water, but once he was given the goggles, lifejacket and tube, he had a great time.
I would definitely price out when you get to the resort how much a cab to Xel-Ha would be (ask the concierge) plus the tickets to Xel-Ha (current prices are on the Xel-Ha website) and see what the difference in price would be. For the 2 of us, it was cheaper to take the tour because paying for a taxi for 2 wasn't worth it, but with 4 in the car, it might be better to take a taxi then you won't have to sit in the tour bus for possibly 2+ hours each way while it stops at a bunch of other hotels dropping off/picking up other guests.