Since I just spent some time off in the Yucutan, I thought I'd give the Grahams my notes on where they might want to go while on their Honeymoon. In case anybody else is headed in that direction, here is some information.
Getting to Cozumel
The ferry leaves from Playa Del Carmen on pretty regular intervals. It's incredible easy to get there. Go to the east end of Ave. Juarez (the busiest street that runs through Playa) towards the Caribbean. The cost of a round trip per person is about $15.00 USD. There is plenty of information available once you are in the area and you should not have any problems getting to the island. Just make sure that you know when the last ferry leaves from Cozumel back to Playa Del Carmen or you'll be stuck in Cozumel for the night.
The ferry will drop you off in the main area of Cozumel. There you'll be able to do tons of shopping like we discussed, mainly silver/gold shops, T-shirts, and cigars. Cozumel is a wonderful place to go exploring especially if you can get there somewhat early in the morning to explore the island or right around sunset to enjoy the dining and nightlife. The taxi services there are pretty regulated, so don't worry too much about getting ripped off. Obviously be aware that it happens, but you should be okay.
If you get there early enough it might be fun to rent a car for just a few hours (it shouldn't cost more than 20-30$). You'll be on the west side of the island when you get off of the ferry. There is plenty to do in that area, but there are also a few hidden treasures on the east side of the island (though no beach). I marked off a bar/restaurant that you might want to look into over on the south east side of the island. During the day it is a beautiful bar that over looks the Caribbean; Mexico at its finest. Albeit you'll need a car to get to the bar because a taxi ride would be very expensive. It's about a 45 minute car ride.
I've listed off where the drop off for the ferry is. From there you can make your way into the shopping area. If you do happen to rent a car, the bar (where I promise there won't be more than 6 people at) is to the south east. The black inside of the island boundaries is more or less the road. Cozumel is not terribly huge, so exploring the area should be a good time.
If you think to do so, look into any snorkeling that might be available. Either at Chankanab Reef, or Palancar Reef. The locals will certainly know what you are talking about. But I am not sure exactly where they will let you snorkel.
This is the mainland. I just wanted to point out the major points so that it's in print for you. You'll see Cancun to the north, which will be to your left when they start driving you out of the airport. The driver should take a right onto HWY 307 to head south. HWY 307 is the road that runs the entire length of the Yucutan. You'll be exiting just a little ways before Playa del Carmen, but all the same, Tulum, and Akumal are also to the south past Playa as you can see on the map. The distance from where you'll be staying to Playa del Carmen is just a few minutes (I think). It will then be about 30 minutes to Cancun (to the north), about 45 minutes to Akumal (to the south), and about 75 minutes to Tulum (to the south).
As I mentioned, "Combis" are available for just a few pesos at most parts of HWY 307. They are little white vans that will pull up and let you hop in. Just tell them where you are going. On HWY 307, you'll see that there will be bus stops here and there for them, otherwise, you can just toss out a thumb when you figure out what kind of car you are looking for. I know it seems a bit sketchy, but they're actually very common and effective.
Another good way for getting around (mainly because its so cheap) is to use the bus system. There is a huge bus station around the corner of 8th Avenue and Ave. Juarez in Playa Del Carmen. Any cab driver can get you there. Anyway, the MAYAB bus service is fantastic and buses are departing regularly to anywhere that you'll want to go. Remember that all of the prices will be in pesos (so you'll find that 8 pesos is really cheap).
One more note of interest regarding money: all ATMS will dispense your money in pesos! This means that when it says withdraw 1000$, that is really only about 100 USD. So keep that in mind. I once made the mistake of taking out just $20 when I really wanted $200. The card worked the second time, but I had to pay twice the international fees.