I agree with Gary. Time is not sufficient to travel from Budapest to Vienna and back in one day and at the same time visit the beautiful capital city of Austria.

I suggest you do a one day trip into the Hungarian Puszta or eventually to Lake Balaton. The Hungarian Railway Company MAV will certainly be able to help you in that www.mav.hu.

A trip on the Danube with a boat may also be a great thing to do.

Enjoy your trip.

Ramón