Vienna is incredible. We’ve spent various holidays there, most recently a week at October half term with kids ages 7 and 4. We walked miles. You can get a 24 hour ticket for the underground which is great value - our 7 year old needed her own ticket, little one was free. The zoo is incredible. It’s a full day on its own, behind Schonbrunn Palace. The Palace is another full day; one wing of the Palace is a children’s museum - it used to be the old nursery rooms, and our kids loved dressing up, wearing and doing the hair on wigs, laying the table, etc. Other highlights are the cathedral catacombs, the wheel at the Prater fairground (which has featured in various films), the Kunsthistorisches Museum, a horse & carriage ride, various playgrounds around the ring, some amazing grand cafe (we liked Cafe Sperl and Cafe Centrale), the Hofburg, and get tickets in advance for a rehearsal at the Spanish Riding School. Eat Sacher torte at Hotel Sache. And the just walk miles and soak up the atmosphere. In 3 days you will scarcely touch the surface, it’s such a lovely city! We’ve also done day trips out by train to Sopron in Hungary and Bratislava, and last time got the train out to Melk (the amazing abbey!) and then got a boat along the Danube to Krems and got the train back, which was a super day out. But in 3 days I’d just stay in the City and make the most of it.