The Basilica of Sant' Apollinare Nuovo costs about €7 to enter and see the entire building and the amazing mosaics. If you like old churches and interesting architecture, this is a must see.
For those interested in historical figures, the town also contains Dante's Tomb author of the Divine Comedy. It is less than a 1 km walk from the Basilica.
Tickets and reservations (http://www.ravennamosaici...)