My young friends (in our twenties) and I stayed at the Park Royal Cozumel for a few all-inclusive nights right before hurricane Dean hit. (In fact, we were evacuated a day early and were sad to go.) Because it was off-season at the resort and because of the hurricane, we almost had the place to ourselves. We loved it and will return.
Food: The resort has three restaurants (two required a dress code and a daily reservation and the third is a buffet) plus a couple snack bars (hot dogs, pizza, nachos). OK, being realistic here, the food is kind of like the food at any other resort: it ranged from just so-so/passable (coconut grouper, pizza) to good/great (fajitas, nachos, hot dogs, dessert, fruit, salads, breakfast). However, there is enough to choose from, and it all seemed fresh and nicely presented, so you're sure to find something you will enjoy! We ate far too much!
The Beach: You get a ticket to check out nice big towels at the beach, and you must return your towel by 7pm or you get charged. The resort is right next to a cruise ship dock, so be prepared for a ship or two always in the background. There are two infinity pools on the beach which are wonderful for grabbing a drink at the swim-up bar. The only thing that would make the beach nicer is if they brought you drinks on the beach; you have to go to the bar for them. The beach is nice but it isn't large, and I can't speak for how crowded it might get in the peak season (Jan - July).
Attire: Bring some comfy shoes to take down to the beach. The paved walkways are sometime bumpy. Dress code for the two restaurants requiring reservations include pants and shoes (not sandals) for gentlemen and a skirt or pants for the ladies. However, I saw people dress more casually without an issue at the restaurants.
Activities: At the resort, we participated in BINGO, and one of the staff kindly gave us a brief salsa lesson. Snorkel gear is free for up to two hours a day, and you will see some fish right off the beach at the resort. Though it is advertised, there is not and has never been a kayak to use at the resort. However, there is plenty else to do. Outside the resort, instead of doing an excursion package, we took a taxi to the San Gervasio Mayan ruins. The cab ride cost $60, and admission to the ruins was about $7/person. It was not as impressive as some of the ruins on the mainland, but it was definitely worth seeing! Also, we intended to check out Golfito, a mini-golf place downtown, but it's about $7/round of putt-putt.
Tips: We were surprised that there was no gift shop or internet cafe; however, I saw people on their own laptops, so there must be a signal. Also, while there is water available at all the bars, there is no bottled water -- only cups; bring a water bottle just in case! Speaking of tips, while you may be on an all-inclusive trip, keep in mind that tipping is still appreciated. $2/day for the maid, $1 per piece of luggage for the bellboy, a few extra dollars for the cab driver, etc. are greatly appreciated. Bring lots of small bills just for tipping.
Summary: All in all, the staff was wonderful, the rooms were perfect and inviting, and the grounds were well kept. We felt safe and relaxed. There are iguanas and peacocks that roam the resort grounds, and it is a relaxing and enjoyable environment. Hope you enjoy your stay!