Ideas for destinations between Budapest and Venice

Hi! My family needs to travel to Budapest mid August. While not an idea time due to heat and crowds, we have a family event to attend. My husband would also like to visit his brother near Venice, so are considering flying home from Venice early September. I know it’s a bit random to start in Budapest and end in Venice, but could anyone recommend some itinerary ideas or destinations between that may not be super hot? Will be with a 6 and 1 year old. We are open to anything - car or train, mountains or sea, adventure or relaxing. The trip is rather last minute so I won’t be picky!