Just returned today from the Royal Caribbean and I wish I could go back tomorrow.
We used Cancun Valet for airport transportation and the cost for four of us was $35. I booked it about three days before our trip and the driver was waiting with our name on a sign when we got there. Be sure to keep walking past some official looking guys with photo ID badges that will try to side track you to their transportation. Walk outside to where guys are waiting with names on signs.
We took a taxi back to the airport for $20.
I'd rethink the idea of going to Wal-Mart. We took a bus there and took a taxi back, but I'm not convinced we saved that much money compared to the grocery store right there at the Royal Caribbean. The food is very reasonable, both at the store and the restaurants. The carryout deli is great and you can get nice, fresh salads, roasted chicken, meatloaf, lasagna, and other stuff and eat it right in your villa. They'll deliver it for free. They also deliver pizzas made onsite and they're good.
When we went to Wal-Mart, I tried to think of what we might eat for the week, but we ended up not eating everything we bought. I left an unopened box of cereal and a package of frozen strawberries for the maid. I think if I'd just have bought stuff as we wanted it from the RC market, we'd have come out much better and not wasted stuff. When you factor in the taxi back from Wal-Mart and the time spent shopping, I don't think it was really a savings.
We did enjoy the shopping trip and saw more of the town on that first bus ride, but we did subsequent bus rides and saw the same things later in the week.
The La Isla Mall was a nice place to walk around and we ate at Las Margaritas and Chili's. Chili's is just like at home, but the great thing is free drink refills. Most restaurants don't give free drink refills and at $2 a pop, the bill can run up quickly.
Be sure to go to the Sunday orientation. It's not a timeshare sales pitch, but will give you a lot of good information to plan your week's activities.