Hi txg8r-
Ditto most of the other replies so far. Chichen Itza is fantastic, amazing, and totally worth the trip- if you have 2-3 days for it. John and donooo are both correct in their assessments, and my 2 cents is: save Chichen Itza for later- I've been there lots, and the best time was when we stayed at a hotel near the entrance and were at the ruins at dawn, before the bus tours overran it. The evening light show is pretty cool, too.
Better for you to go to Tulumn- great ruins in an absolutely stunning setting on the coast. Go early, take your swim suit (there's a terrific little beach right in the ruins), take lots of film and a picnic lunch (film and food are an overpriced rip-off at the park, although there IS a Subway with decent prices- but did you really come to Mexico to eat at Subway?) and, more importantly, try to read up on Tulumn a little before you go (you can find English-language guidebooks downtown Cozumel, or research it on the internet before you go). You'll enjoy the ruins a lot more if you understand what it is you're looking at. (One of my "cheap trip" suggestions is to quietly "attach" yourself to one of the many groups, mostly from the cruise ships, you'll see touring the ruins with a local guide and listen in for a no-cost guided tour. But do your karma a favor and tip the guide $3-4 discreetly at the end!)
As for getting there, I beg to disagree with donooo. There's really no need to rent a car or Jeep for the day just to get to Tulumn, unless you have the $65+ to throw away or just like driving. There are lots of local buses that leave frequently from the Playa del Carmen bus station (2 blocks from the ferry) for only a few dollars. It takes about 25-30 minutes, and they drop you off on the highway right at the entrance to the Tulumn park (there is a tram from the entrance to the ruins themselves, but it's not that long to walk), and you can catch the bus back in the same place. You'll have more fun, mix with "the locals", see the same sights along the highway (not much to see, really), and save $50 or so to spend on diving or, better yet, a great meal and a margarita back in Playa while waiting to catch the ferry back to Coz.
Whatever you decide, have a great trip and post a report when you get back!
FastDen :rainbow: