Some of the comments that say "go to Tulum, not Chichen Itza", are way off the mark. All of us will probably got to Coz 20 - 30 times in our lifetimes, so plan on going to both. Ya know, one one time then go to the other place next trip
Tulum is quite neat, not the spectacular ruin like Chichen Itza, but it has it's own interesting topography and stuff to see. At either site, get a guide. Some of those "piles of rocks" are very interesting if you have someone that knows their story telling you about them. Important safety tip, Tulum is free on Sunday. Like many have said, it's about an hour S of Playa.
Take Christi or Shark's advice about going to Chichen Itza. I once took an early Ferry, grabbed a "rent a racer" and drove to Chichen Itza. Spent about 2-3 hours at the site and drove back in the same day. That nite, my tongue was hangin down below my feet.
By all means, go to Chichen Itza tho. it's awesome with the huge sacrificial Cenote, the huge Castillo, the ball court and all the other ruins. There are really cool statues, heads, and other things just laying on the ground. It's an inspiring place.
There's another fun thing to do. On the way back from Tulum there are little gravel driveways on your right. Some have signs saying "lunch", "cerveza" or something. go down these roads. All of 'em have a cenote you can snorkel or just swim in. they all have great lunches and usually a great crowd of friendly people. most of the people that hang at these are divers on their off day, or hiker/backpackers blowing off a little steam. We call these places "Cenote bars". In the old days Xel ha & Excaret were Cenote bars. now the tourists and pod people go their and trhe cool kids hang at these Cenote places.
There is another place that we go to that I haven't seen discussed. there is a pyramid at a town called Coba a little S of tulum. There's just a few people that go there and it gives you a look at a castillo that hasn't been cleared off yet. Then on th way back, do the Cenote bar tour.
Also do San Gervaiso on Coz. It's a small ruina, but it has it's own story to tell. go there, then head to Coconuts and the Raggae Bar.