Introduction
we had clearly overestimated the time we would need in Vienna and had bargained on a few excursion. Our preference would have been to do the Grossglockner Hochalpenstrasse but that will have to wait for another day as it was too early in the season.
Vienna to Budapest - train, rent, cruise or walk
The easy part of this question is that you cannot walk between the two cities but the other options are certainly available. If you have the luxury of time you can cruise along the Danube from Vienna to Budapest. I'm sure you can do it the other way but probably against the flow of the river.
The two options are train or drive. Train tickets for individuals or couple will probably work our more economical but for a family a rented car seems to be the best option. Car rental from Vienna allows you to cross the border into Hungary. The one thing you will have to watch out for is to pay the registration fee for you car to cross into Hungary. This is done at the border or one of the service stations before the border. Once you do this its a fairly straight forward journey.
Good maps and navigation are useful for city navigation but the road between the two cities is pretty straight forward. We picked Budapest but you can equally try Bratislava or Prague. Although Prague seems to be a little bit further.
The road between Vienna and Budapest is fairly boring and its a straight drive. We wanted to start early but changing currency getting used to the rented car and the wrong side of the road took a little time. But once on the motorway it was fairly straight forward. The journey time was about 3 hours. we arrived in Budapest around mid-day.
Walking around Budapest
We took the sightseeing bus around the city and it came with a evening cruise along the Danube. This is particularly worth doing as the city come to life in the evening.
Budapest at Night
Budapest really does come alive at night, most package tours include some sort of evening boat tour along the Danube. You can pitch along the wealth spectrum with tourist boat with formal sit down dinners to pizza and loud music boat tours, If you're on a short visit to Budapest then the day bus tour packages come with a evening boat tour (the pizza and loud music kind). You should not let this diminish the beauty of the evening on the Danube.
We were only in Budapest for the day but its clear to see that its a wonderful city to tour during the day and evening. If you can stay longer then its even better.