Calais to Venice by car. Stopover?

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I'm trying to plan this years holiday. I only have 16 days (15 nights) including travel, with kids aged 4-11, and want at least a week on a campsite near Venice. It's a 6 hour drive to the ferry then 13-14 the other side. Dh and I can share driving so could do 1 or 2 stopovers on the way and on the way back. Any suggestions where? Or for how long? I'm thinking at least one stopover should be a few nights do we get to see a bit of somewhere on the way there and back. I'd also like some space for kids to burn off some energy, or a pool or something, after a day cooped up in the car.

We drove to Florence. Did a 3 hour drive after Calais to Durbury in Belguim. Very pretty town with lots of activities in the big river. Leisurely drive to Germany to see concentration camp ( stayed the night, visited in morning) drove through Austria and got to Florence by 8pm.
Did Milian on way back to see The Last Supper. You need to book in advance but was totally worth it. Stayed in a nice hotel on the outskirts with a pool and got the metro in.
I'd say pick some something you'd like to see enroute do half a day driving and enjoy the rest of the day wherever you are. It wouldn't take more than 3 days each way.