I recommend the Vienna to Budapest 5-night tour, especially as an inter-generational adventure. We were a group of two 30-somethings and a 72-year-old dad, and it was a great fit for all of us. This is really a premier section of the full Eurovelo 6 cross-Europe route. I was especially impressed with bikeways and great signage all along the route. We were impressed by the variety and quality of the hotels, and the destination town or city at the end of each day was well-chosen. It was such a treat to go exploring each evening after getting off the bikes!
Squeezing it all into the four-day sporty/active itinerary, however, was challenging on heavy cruiser-style rental bikes, so I'd advise either slowing it down or bringing your own bike.