An obvious mid-point is Verona, which is by no means off the beaten track. It's a pleasant enough mid-size city, and enjoying an outdoor opera in the Arena is spectacular. (You can safely skip Juliet's balcony, though.)

But my suggestion would be Ravenna, not a bad drive from Milan and 90 minutes south of Venice. In Ravenna, you'll find mosaics from the 7C/8C that simply lifted me in the air. [Which is not such an easy feat.] I'd grudingly gone along when my [then-]wife added Ravenna to the itinerary, but I'm incredibly glad that I didn't miss this hidden jewel.

If you do go to Ravenna, note that you'll find some accommodations downtown, but most accommodations in the tacky beach section a few miles outside of town. Avoid the latter.