The canals are interesting and worth a boat trip,at least once.
You can travel on your own cheaply by getting a day ticket for the local trams.
If you want to go further afiels it is easy enough on the local trains--the main railway station is only 10 minutes walk from the boat docks.
The cathedral is magnificent.
The Roman museum is one of the best I have seen.The large intact floor mosaics definitely the best we have seen.