Valladolid is a very pleasant small city and a great day trip from Izamal.
Originally built in 1545 and home to the San Bernandino monastery, Valladolid has some very impressive colonial architecture and the vendors in and around the main square offer excellent local handicrafts. Valladolid is also an important place in Mayan history as the Caste Wars began here in 1849. The town is built upon the Mayan town of Zaci, which is also the name of the local cenote. Nearby are other cenotes, probably the most notable being Dzitnup. You may also want to double it up with a visit to the ruins of Ek Balam.