I typically pack pretty light. For an 8-day trip to Cancun/Playa del Carmen I'll bring a couple pairs of jeans, shorts, a simple dress, bathing suit, couple light sweaters, couple light tops, runners, sandals. Even though the hotels offer toiletries, I also bring one of those travellers packs with a mini toothbrush, etc. Thinking I'll buy "Off" and sunscreen at the shopping centre in the Hotel Zone because I can get full-sized ones (rather than packing minis).
Of course, like most people, I also like to pick up one or two things on my trips as reminders of the trip. Nothing big, just something kinda cool.
If you were going to suggest one thing you found a "must have" that you would pack for Cancun/Playa del Carmen and a "must buy" from Cancun/Playa del Carmen -- what would those things be?