Coba is an ancient Mayan city which is about 1 hour from Tulum. Coba is a small town but with a lot of history. You can find old Mayan pyramids which is incredible to discover and also you can find a lot of cenotes, which is underground water pools. These cenotes were used to supply the Mayans with water but now you can visit them and swim in them.
There are a lot of different cenotes to visit in Coba, some more touristy than others.