I've been to Playa about 15 times or so. For dive op, It depends on what you prefer, big or small. I like small, so Fantasea dive is by far my choice, Jason is awesome. As far as other activities, besides what Ron mentioned... When you dive Playa, make sure you hit Punta Venado, it's hands down for me the best dive in Playa. The Mama Vina is a great wreck dive right nearby too. A lot of the big ops wont go down south that far, but by panga it's only about 20 mins and well worth it, Jason will take you there if you set it up.
Puerto Moreles - Nice relaxing uncrowded beaches, great ATV/Cenote (non-diving tours)
Akumal - Half moon bay snorkeling, Yal-Ku lagoon (cheaper version of Xel-ha, go early at 7/8 when they open

Lots of turtles in Akumal and half moon bay. Also, if you enjoy snorkeling, grab a panga out of Playa and tell them you want to go to the Punta Venado beach area

. Great snacks and view at Buena Vida in Half moon bay.
Xel-Ha - Massive snorkeling park...Pricey and can get real crowded, do Half Moon Bay and Yal-Ku
Tulum - El Pariaso - Great beaches and beach club, for us WAY better, and less crowded than Playa. Make a day of it, Ruins first thing in the AM before all the buses arrive, then the beaches till 2pm. From there on the way back to playa stop by Casa Cenote. Great cenote and restaurant right on the beach.
To add to Ron's Coba - Be there right at 8am when they open, rent a bike, and off you go. You'll be done before the mobs in the buses arrive

On the way back hit Grand Cenote

. Last time I did Coba, I was there at 8, up the pyramid and had the area to myself for about 1/2 an hour... it was an awesome experience.
Playa - Stuff on 5th worth checking out...
Tacos - Carboncito's - best taco's in Playa IMHO
Drinks - Tequila Barrel - Great for people watching

Dessert - Playasia - 2 words.. tempura cheesecake...
I'll add some more later as I have more time, at work now.
Greg