Rick, I can't agree with your comment about the car rental agencies. We have been renting cars in Ca

Hi - We will be traveling as a family (2 adults, day tours from london kids 13 15) for the first time to Playa Del Carmen for December break. We are staying in Playa Del Carmen Saturday to Saturday but arrive in Cancun on Friday.
2. In Playa, we will want to eat in town almost every night and don't mind the 15 minute ride to town. Should we take a cab or rent a car? We usually rent a car (St. John, TCI) but are just a little unsure of the safety of foreigners driving in Mexico.
In Cancun you won't find something extraordinary, so I would recommend you going straight to the lovely town of Playa! There are a lot of small hotel in the town center and very close to the beach so no needs to rent a car Furthermore taxis are very safe. If you want to do day excursions, just find one of the many local operators in the town center, it will be much easier for you.
We have been going to this area of Mexico for many years, but have no desire to stay in Cancun. If you want to stay somewhere for one night before Playa, I would recommend Puerto Morelos, which is a small town between Cancun and Playa with a nice beach and some small hotels and restaurants. I'm not sure I understand '15 minute ride into town'. Are you staying at a resort outside of Playa? If you are staying right in Playa, you won't need a car as you can walk everywhere. However, having a car for at least a few days is a great bonus. This will allow you to explore day tours from london places like Tulum, Akumal, Puerto day tours from london Morelos, etc. Driving this part of Mexico is very easy and safe, except for the traffic in downtown Playa.
We have yet to drive in or to PDC (bus to PDC, feet or taxi in and around PDC). That said, we have subscribed to tours to get to Tulum and Coba. In retrospect, a rental car probably would have been more efficient both in cost and time (our bus tour to Tulum stopped at multiple resorts to and from to pick up other tour members).
If you are going for Christmas/New Years week, you need to book accomdentations now, if it's not already too late. Many resorts day tours from london fill up fast and sell out. Some also require minimun night stays. That is also the most expensive time of the years as rates are terrbily high and for some prohibitive.
I have never found the need to rent a car in that area. We tend to stay right in the 5th Avenue area, so everything is walkable. Taxis are reasonable and plentiful. For off property excursions we do tours or take a taxi.
My personal favorite place is Playa del Carmen. Not at one of the beach resorts day tours from london - right in town. We have been staying at El Taj in Playa: right on the beach, and a block from Fifth Ave. As someone mentioned, you don't need a car (and the car rental agengies are neither reliable nor honest), but you do need to make your Christmas reservations right now (we made ours about a year in advance). As far as getting out to Xcaret, Xel-Ha, Tulum, day tours from london Coba, Horseback riding, jet-skiing etc. All of that can be arranged at the El Taj front desk (at least they did it for us), and the arrangements include transportation. But watch out for a day trip to Sian Ka'an - it's not for everybody. I've written quite a few articles about Sian Ka'an and Playa, and they are on my website (check my profile)
I agree with everyone else as far as skipping Cancun. Where in Playa are you staying that has a 15 minute ride into town? Why not stay right in town. There are many hotels and some nice condo rentals that work well for families. You can walk almost everywhere. You only would need a car if you go on an excursion, although even many of the sites can be reached from town by the public colectivo vans or taxis.
rickhowe, I read you review on the Sian Ka'an Biosphere. Though day tours from london I agree with your description, I don't agree with your thoughts. We loved our tour and thought it had the best snorkeling of any of the other places in that area we've been to. Yes, it's a very long day and that road was eternal and bumpy, but the end result was worth it. You are probably right though that it's not for everyone. It was an adventure.
Rick, I can't agree with your comment about the car rental agencies. We have been renting cars in Cancun and or Playa for 8 years and have never had a problem. One time we had to wait 45 minutes for the car to be ready, but that was the only issue we ever had. All in all the smaller agencies like Easyway and America are easier day tours from london to deal with and are far cheaper. I'm sure there are some horror stories out there regarding car rentals in Mexico, but that can happen anywhere.