Day Trip Vienna to Budapest

I'd really suggest spend a night or two in Budapest. A day trip is really just not enough to really see the city.

The train ride is rather boring as it rambles slowly through river delta farms that look like Eastern Arkansas.

Well i think it's possible to see almost all the popular spots in budapest within one day. Stayed there for more than a month and when my friend came over, i brought her to see everything within one day.
suggested itinerary:
8am- fishermen's bastian/buda castle area( honestly theres nothing much to see, the castle area is just an area where theres a lot of museums), you can take a tram down to the river
10am-stroll along danube river
11-1pm- walk across the bridge to the Pest side to the Central Market which is located just beside the bridge. you can eat lunch and find local food and groceries there
1pm-3pm-take a train to Hero's square and explore the area, the Szechenyi thermal bath is very near it, you can stay there for a couple of hours if you guys are quick with previous stations
4pm-you can take a train/tram to the Parliament House

Actually that's pretty much it. since you're leaving at 9pm, maybe you can catch the sunset but Budapest is much more beautiful at night with all the lights, its simply magical at night.

I agree that if you can spare a night to stay in Budapest. The boat cruise to the Danube Bend is wonderful and the view coming back into Budapest is spectacular. To be able to wander around in the evening and see the city lights without having to rush off would be a real bonus. I love Budapest and think it is worth the night.

One day in Budapest in December? Please rethink and take at least two! We spent three days following a Danube river cruise several years ago, mid-December to mid-January 2013-14 and just spent a week there in June (we have Hungarian friends so really know the city and surrounds!). this past trip took the Hop On-Hop Off bus tour(Green bus is best) for the first time since my sister was visiting also and it gives a really good overview, then return to stops for exploring. In December the Christmas Markets are in full swing and the city lighting is enhanced with all the additional holiday lights. (We also took a two day express train trip to Vienna to visit the Christmas Markets there, wonderful as well!).
Visit at least one of the thermal bath spas and book a massage; Kira'ly, Gellert and the largest (near the city skating rinks) with the outdoor pools open in winter are each quite different and wonderful in winter AND summer. On our holiday visit we also visited Lake Hevitz, a famed resort and hugely popular with many Eastern Europeans.
My very favorite monument is Heroes' Square as it represents the unification of the tribes into the state of Hungary and all the Hungarian kings, ending with the takeover by the Austrians, then of course, Germany, then Communism....but now independent again!
At the base of the funicular up to the castle area is a huge sculpture "0", which marks the beginning point of all KM readings throughout Hungary!

Visit the 2nd largest synagogue in the world, the largest is in NYC. This visit we had a special guided trip of the old Jewish sector/ghetto which is now full of special artists, boutiques and "ruins"(bars and pubs rescued from demolition and hugely popular with the hipsters and students of Budapest). Also tried the fare at traditional and kosher coffee shops, bakeries and restaurants.

Get off the main tourist streets into side streets and have real Hungarian foods; yes, hearty, but it's winter! Upstairs at Central Market have a Langos.....the very original is topped with garlic, but they offer all different kings of savory and sweet toppings!
Have a good visit but do slow down and take some extra time to really get a sense of the city. We are now in the final weeks of our 2nd four month long road trip exploring different (for us) areas of Europe and spend 5 to 8 days in an apartment, guest house or B&B where we can get acquainted with our hosts and the areas. Currently outside Caen in Normandy and will ferry from Calais to Dover for several weeks in the English countryside. After seeing the Bayeax Tapestries and William's castle now must see !Hastings and Battle where her became William the Conqueror!
Now we have the time to be able to stay for an extended trip and it sure beats a two or three week "tour"!

I also found Heroes Square pretty fascinating, although very esoteric; actually, it was the very first place i went to after arriving at Keleti. Don't forget the takeover by the Turks...Mohacs came first, then Karlowitz, which expelled them for good.