Tulum offers the ruins and a passle of beautiful (if sometimes primitive) beachfront living. But it might almost be more valuable as a sort of hub from which to explore other "Mayan Riviera" spots.
The ruins at Coba are only 47 klicks out of Tulum, and, even without the dropdeap stunning beachfront real estate, are much more impressive. Easily reachable by highway bus or even taxi. You could even hitch, actually.
The limestone bedrock on the area (in fact, of the entire Yucatan) is riddled with sinkholes, which they call cenotes. There are dozens of them near Tulum, many on the highway with "Another Roadside Attraction" signs pimping in business, some hidden out in the jungles, some the site of quiet restaurants with diving available. Very recommended--diving in the still, shaded, green waters of a cenote is very different from sea diving. The very setting has a feel to it. You can easily see why cenotes were revered the Mayans. (And from there it's just a jump to realizing that it would be a good idea to kill virgins and dump their bodies into the water laden down with gold idols--you just sort of get that feeling). These are really cool places, and they are all around. Dos Ojos on the highway to Cancun is pretty fully developed and has a diving/snorkeling program. The Grand Cenote is on the road to Coba. Cenote Manate is on the coast just north of the Ruins. If you become a cenote freak, you will find all sorts of willing guides.
The various "ecoparks" are all within a quick bus ride, or you can visit the Sian Ka'an Biosphere, which actually IS an ecological park.
In face, it's only 63 Km. up to Playa del Carmen, if you wanted to check that out. Tulum makes a pretty good base of operation for people comfortable with beach lodging in the $20-30 range. And Xcaret, a huge "nature" disneyland place with everything from snorkeling on reef or underground river to jetski "ecotours" and a sound and light night spectacular, is right up that way as well.
It's a mere 15 Km. to Xel-Ha, which is more my idea of a good time. This is an incredible place, where spring water bubbles out through undeground caves and tunnels, mingling with the ocean in a big estuary. This might be the ultimate snorkeling anywhere. Xel-Ha has become an "Eco Park" lately, sort of the king of them, with maybe Xcaret as a rival. For more comment on that, see my entry on that subject.
You can also get about any kind of diving tour (sea or cenote) you can think of or check out the Casa de Cultura. Ask around, especially in the Weary Traveler in town. You can laze your brains out on the beach here, but it's also right in the middle of some world-class interesting stuff.