Budapest, the capital of Hungary, is situated about 3 hours driving from Vienna. And it's probably the first thing to consider when deciding to take a day tour to visit Budapest from Vienna. You should expect about 6 hours of driving, 5 hours of sightseeing and free time, and with a few comfort stops along the way, makes it an about 12 hours long tour.
The city of Budapest has quite a lot to offer to it's visitors, even if you stay for a few days. But if you have just one day to spend, the several hours that you will have there will be enough to see the highlights of the city. Among many attractions that the city has to offer, you will definitely visit the Hungarian Parliament - the 3rd largest parliament building in the world, the Heroes’ Square - the famous square with the impressive pillar with the statue of the archangel Gabriel who holds the Hungarian Holy Crown, the Fishermen’s Bastion - a extraordinary terrace with an amazing panoramic view of the river Danube and Pest and the Széchenyi Chain Bridge - the first permanent bridge connecting Buda and Pest.
If you decide to take a day trip to Budapest from Vienna you can expect to see the highlights and the most important parts of Budapest, and hear the short version of the history of the city. Maybe this will be enough for you or it will make you want to come again and explore this vibrant city even more!