The Cathedral and tower sit on the Piazza Grande so they can all be seen in one go. The Cathedral is the main sight and is a fine piece of Romanesque architecture. The interior of the cathedral is quite impressive with sturdy walls, the interior splits to two levels divided by the impressive tribuna. There was a service going on below, so I was not able to get to see the coffin of San Germiniano to whom the cathedral is dedicated. There is a very impressive mosaic above filling the rear niche and providing a focal point for the cathedral.
The Torre Civica is on the rear and is obviously leaning, it is accentuated by the lean in the opposite direction by the walls of the cathedral itself. It is possible to climb the tower but the opening times were a bit erratic.
The Piazza Grande is included more for the cultural/ historical relationship than its impressive architecture as there were not any real buildings of note. Modena seems like a nice city, quite small but probably has enough to keep you there for a night if you wanted to go out and see Ferrari's factory at Maranello about 15km away. Modena is ideally situated on the main north south rail line between Milan and Ancona, or could make a nice day trip from nearby Bologna.