Itinerary Help Between Budapest and Venice in August

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, and quite the distance in between… but could anyone recommend some itinerary ideas or destinations between? Criteria include comfortable temperatures (not too hot), kid friendly, and activities such as mountain hikes and gondolas, or water destinations at beaches or lakes. Willing to use any form of transportation including car rental, train or flight.