This city was the most important capital city of the Maya area, and is now part of UNESCO World Heritage. Chichén Itzá is 40 min far from Valladolid. They open from 8am to 4pm. I recommend you to be there early, just before the bus tours arrive. It won't take you more than 1 hours if you are not with a tour.
2. Ik Kil
Take a jump into the fresh waters of Ik Kil after a hot morning in Chichén Izá. This cenote was used for sacred rituals. It is beautiful, but I am not going to lie, if full of people will feel like in a public pool.If you want to avoid that feeling, try to get there as soon as they open the doors. I believe the entrance is around 70 pesos.
3. Samula and X'quequén
These two are 10 minute driving from Valladolid. It was quiet and dark. Light only pass through a small hole. Beautiful. You also have Zaci in the center of Valladolid.
The experience is totally different than Chichén Itzá because these are in the jungle. It is totally worthy to rent a bike, or have a ride. This archeological area is located between two lakes. We visited Punta Laguna, talked with real mayan people and saw monkeys.
5. Theme Park
There are several park around Riviera Maya you might want to visit. Check also their night cultural shows.
6. Tulum Archeological Center
The view are amazing, but it was so crowded. This ruins are really close to Tulum city and the hotel zone. Explore the hotel area, there are good restaurants, boutiques and amazing beaches.
7. Akumal Beach
Tulum is a great place for snorkeling. In this beach in particular you get the chance to swing with turtles. No need to pay for a tour or a guide. You will find them, guarantee. We got breakfast in one of the hotels with ocean views and spent all morning in the beach.
8. Playa Del Carmen
If you are into shopping, Playa del Carmen is the best place for it. You can also contemplate the "voladores de papantla" or hang out in this beautiful beach.
9. Cenotes Riviera Maya
There are a lot of cenotes that you can choose from. Grand Centore and Dos Ojos are well known, but is only worthy if you want to do scuba. We went to AktunChen. It was all for us :)
If you have one extra day, you can alway visit Cancun. Isla Mujeres or Cozomel are hot spots. You can also go out or just explore the beaches around the are.