Day 1: Chicen Itza and Tulum
Chichen Itza is readily accessible as it's in the middle of the route from Tulum to Merida, and numerous tour buses make the 1-2hr trip there and back to either ciy every day. While it is certainy impressive, my opinion of it is a little muted due to the fact that Uxmal is more detailed and significantly less busy. Another item of note is that the main pyramid is not entirely original - the exterior underwent significant reconstruction. The site itself is nevertheless quite interesting, and is worth a stop over while travelling from Merida to Tulum, or as a day trip from either. After all, it's not every day you get to see a Wonder of the World, even if the pyramid is the least impresive of the four I've seen as of writing.
A addition to Chichen Itza, Tulum is an archeological site in its own right, and it's got a waterfront view to boot. To be totally honest, I think I may have been burned out on ruins by the time I reached Chichen Itza and Tulum, or maybe it was the unending crowd coming down from Cancun and Playa del Carmen, but Tulum was not one of my favorites. Nevertheless, the ocean views give this site a very unique backdrop.
Speaking of ocean views, did I mention the beach?
Day 2 and 3: The Beach
It's full on Carribean sunshine - the beach adjacent to the Tulum ruins is called Playa Paradiso for a reason. I specifically planned the trip to end at the beach after many days of inland travel so we could just decompress, process everything we saw, and come back to regular life fully recharged. As part of my beach stay, I had intended to go diving in Cozumel, but unfortunately the humidity got the best of me and I developed a cold. Nevertheless, I hear fantastic things and would suggest looking into scuba if it's something you're into.