I think that Verona would be a good choice. Lots to see and do in Verona. You could even do a daytrip to beautiful Sirmione and its castle on Lake Garda or over to Vicenza for a day. We went to Sirmione as a daytrip from Verona last spring and took the public bus. It was easy but did involve a fair amount of walking from the bus drop off and pick up point, into the actual town. You could take a taxi into town itself. I was wowed by both Verona and Sirmione. I personally liked Verona better than Padua but that is a matter of opinion. I didn't dislike Padua, I just didn't like it as much as I had anticipated. Padua could also be done as a daytrip from Venice.
You said you wanted something slightly different. You could consider Ferrara. It is on your route to Venice. I liked Ferrara very much. It was very pretty, also has its own castle right in heart of town. Much less crowded. A possible daytrip from Ferrara could be to the small picturesque town of Monselice. (Actually closer to Padua so consider it too as a daytrip if you choose Padua.)
Another option would be to travel to Ravenna on the coast and see the mosaics. All of your options are great choices so you cannot go wrong.