I tried to spot at CUN last May and it was extremely difficult to find a nice spot outside the secure area. If you walked to the left or right of the terminalbuilding, views were blocked by cargo areas and buildings or roads weren´t accesible. I was able to spot some planes far away when they took off.
But there are some interesting aircraft apart from the ones mentioned; it´s a base of Aerocaribe so you will see some Jetstreams, maybe their final and rare Fairchild F-27 on the Cozumel shuttle and some DC-9s. Aviacsa also brings in 727s. US carriers come with interesting equipment, like DL 764s, CO 753s, ATA Tristars. TACA flies 737s. Enjoy your trip!
nobody has ever died from hard work, but why take the risk?