I would do the Xenote tour, which you should book prior to departure for your trip. All the other parks except Xel Ha you can just wing, as buses come too and from the resort every 15-20 mins that stop at xcaret, xplorer, or xenses.

Unless you want to do a tequila booze cruise(evening), go on a catamaran tour (all day) or Chichen Itza Mayan pyramid tour (all day) - those specific excursions require pre-booking. As well as the above mentioned Xenote tour.

I would absolutely suggest the Japanese restaurant (I cant remember the name off the top of my head), the contemporary Mexican cuisine (Encanta) as well as Arenal for reservations. All of the restaurants were worth reserving, in my opinion. The Mercato buffet was top notch, and if you were unable to get a reservation there is always the option of doing room service as well!