Review about Cycling the Danube from Passau to Budapest
Vienna to Budapest is a great trip. Its very easy to cycle from B&B to B&B. Much of it is along the Rhine. Hungary is very cheap and friendly. Its possible to rent a touring bike in Vienna and drop it in Budapest. You can also return to Vienna via a boat on the Rhine. Here is a link to a trip I did. I did not cycle the Passau to Vienna portion. There are others who have and you can find them on the crazyguyonabike.com site.