Sian Ka an Biosphere Reserve, Tulum
As you head down the beach road to Punta Allen, the road gets worse and worse. It becomes a path that kicks up tons of dust, becomes more impassable, and you can't see the beach from the inns and palapas and shacks, etc. After about ten kms from the main road you come to an arch in the road where you are greeted by a guard of sorts. He waves you through, and about four more kms you come to Cesiak.
It is a restaurant/hotel/NGO research center for Sian Ka'an, "where the sky meets the sea". We took an all day tour of the nature preserve, which is a barrier island, a brackish lagoon followed by a canal that goes through mangrove and sawgrass inhabited by migrating tropical birds, baby tarpons, termites and crocs (didn't see any of the latter as they are nocturnal), and finally to a fresh water lagoon. Around that lagoon are mayan ruins and a curious little temple to the alligators.
Our tour was led by two locals who had encyclopedic and scientific knowledge of the flora and fauna, and a university educated young man (whom my wife thought was hunky) who gave wonderful guidance. We took a short time and floated on our lifejackets down the cool current of the canal. Great!
5000 sq kms of tropical jungle, marsh, mangrove and islands, Sian Ka'an (Where the Sky Begins) is a World Heritage Site and well worth a visit. You can do it yourself as the road form Tulum to Punta Allen runs through it, but it best appreciated on an organised day trip as this involves a boat trip through the swamps, lunch on the beach and rubber-tyre aided 'swim' (the currents are so fast you don't have to work very hard!) down a crocodile infested creek (fear not - the boats block the water junctions once ensuring the creek is croc-free!).
One of the best places to organise a trip is at Cabanas de Ana y Jose, the last hotel on the stretch of beach prior to the Reserva.
Punta Allen is a road that runs parallel to the main highway, right along the beach. On one end is the Tulum ruins, and on the other is the Sian Ki'an Bio reserve. Whether or not it is the only road unmaintained by the Mexican government is up for debate, but the road is full of potholes and will work your vehicle. We had a microcompact, and ended up turning around after entering the reserve ($2/car) and driving through it a bit. The road is extremely long, but there is a beautiful beach not far down that is worth driving to if you have the time. We didn't see any wildlife in the reserve, but then again we didn't go to far into it. There are some shantys/shacks at the beginning of the road on the beach, which gives an interesting glimpse into the lower rungs of mexican society.
Visiting the Biosphere is a good way to get away from the boredom of beach paradise, if there is such thing! The Biosphere is located south down the beach road that runs by all the Cabanas. It will take you about 20 minutes to get there from the "T" in the beach road. One of the more exciting activities is kayaking around the seemingly endless waters. While it wasn't the funnest thing I have ever done it certainly was something different and worth the visit, though I don't think I would do it again. In fact the only reason I went in the first place was because I met two girls at the beach and thats what they wanted to do and as usual I gave in because thats what I do with pretty girls, I give in.
We spent 2 nights in Cesiak in the Sian Kaan Biosphere Reserve and rose at 5am to welcome in the morning - "You can sleep when you're dead!". The wildlife at this hour is a joy to observe and the experience was almost holy - a truly incredible experience. I'd only recommend staying at the tent cabanas here if you're the kind of person who doesn't mind roughing it a little though - we are avid campers and it was perfect. But watching the sun rise anywhere along Tulum's beach stretch is incredible.