So much to do, but I wouldn't pack it all in.
You will be back :) again and again with him.
We have done something different every time we've gone to Mexico.
1. Chichen Itza ; Did an all day tour with trip to a Cenote. Was so amazing. The lunch was really wonderful buffet at a nice hotel right outside of Chichen Itza. There was some light entertainment while we ate lunch and enjoyed margaritas. About a half hour away on the way home, we stopped for a 20 min jump/swim into a cenote. Cenote was beautiful and exciting for the kids. Long drive, but easy on the comfortable bus. When I ask my 3 kids what the most memorable thing they have ever done in Mexico - it's Chichen Itza. Must have a tour guide for this because you will miss out on the amazing history and the legends. We got a discounted rate for our family through the resort - did a 1 hour "update/check" of our timeshare at the Westin Lagunamar. We were going to hire a private driver - I am so glad we didn't because the tour guide was essential, the bus was so comfortable, and we were able to sleep on the way back. There were also refreshments on the bus and we stopped a few times for a snack and for 10 min of shopping on the way there. If you are not up for a road trip adventure, or if you could care less about ruins, then I think your plan to see El Rey is just fine. It is very small but will be fun for pictures. http://www.mycancuntours.com/tours/chichen-itza-tour.asp
2. Heard Xcaret is really great with an amazing evening show. We will probably do this with our kids this year.
3. Isla Mujeras day trip - rent a golf cart and cruise the small island. Swim at Playa Norte. Stand Up Paddleboard rental on the beach. Snorkling also on island.
4. Tulum and Xel Ha combo trip - If I had to choose only one, it would be Chichen Itza. Although, Tulum has a beach on site that you can climb down to and take a dip if it is hot out.
5. The beach by our hotel was wonderful and the lifeguard had boogie boards that they rented very cheaply.
6. Something I read about but have not done yet is Pericos in downtown. Supposedly it is very fun and lively with entertainement going on all around as you eat or enjoy a few cocktails. I think it is family appropriate. My husband and I are going to check it out and then probably bring our 15 and 16 year old when we go at Easter.
Have fun!!!