What are the most artistically and architecturally interesting  small
Italian cities or towns within an hours drive of Bologna?

Maybe you could set a limit on the number you need, or if you are
planning a trip, the time you have available.

Yes, that would be a usuful parameter. Thank you. We have three full
days, traveling by car each day out of Bologna at 8 AM and returning
that night by 6 PM.

Ravenna, Parma and Ferrara are all historic, architecturally superb
cities within the same region as Bologna (Emilia Romagna) and within
about an hour by car (50kms or so). Ravenna has churches and mosaics
from the 6th century onwards; Parma one of the finest ensembles of
Cathedral, Campanile and Bapitstry in Italy (rivalling Pisa; and
Ferrara, the home of the Este family, has wonderful Renaissance
palaces and art. However, I hope you leave yourself sufficient time to
explore Bologna, with its magnificent civic buildings, including TWO
leaning towers!

Great recommondations.  Those three towns should fill your three days
and leave you wishing you had more time.
Have a great trip.