Otherwise, there are countless options in Tuscany: Arezzo, Cortona, Volterra... the most "obvious" may be assumed Siena, certainly if you don't know it it would deserve a visit; however, it deserves more than some hours, and visiting it as a stop in a road trip is not easy: all the centre is closed to traffic, and you'd have to park outside then walk a not so short distance.
The problem is that Arezzo is too close to Umbertide to be a place for lunch. Unless you decide to visit Arezzo a bit carefully (the city deserves it); this way the lunch time will come soon.
Another option is to stop in Florence, but without entering the city by car (there is the problem recalled by Aristophane); just park the car along the A1 motorway, at the "Villa Costanza" parking lot, between the "Firenze Impruneta" and "Firenze Scandicci" toll stations, then take the tramway, in just 23 minutes without stress you ride up to Florence center.
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Then, at "Firenze Scandicci" toll station you can exit the motorway to take the highway (no toll) to Pisa.