We were at Cozumel last February. We went to Playa Mia which I think use to be called Playa de Sol before the hurricane last year.
There were 6 of us so we ended up renting a minivan from Executive. When we rented the minivan they gave us coupon to get into Playa Mia for $3 per person. This was a great deal for the 6 of us. Playa Mia charges $14 per person regularly for their basic entry fee and the cruise ship wanted to charge us $29 and we'd have to take a taxi there. This fee includes the use of the beach and all non-motorized sports. The kids had a blast - there was a water trampoline, rock climbing, paddle boats, kayaks, snorkeling (rental equipment extra), kids activities.
Rental equipment inside Playa Mia was a rip-off $12. We bought it at vendors outside for $9. You can pay more for admission and have unlimited drinks and food (I think it was around $40 at the gate - Carnival wanted to charge about $75 which also included your transportation to and from.)