Another building designed by eccentric Austrian artist Friedensreich Hundertwasser (see also Hundertwasser House). This one has an equally outlandish appearance, and houses a museum on his life and work. Check out his wikipedia page - He was a cool guy.
You can go inside the building for free, which is a really nice thing to do in itself , and maybe get a coffee in the gorgeous cafe. The museum part costs €11.
Getting there: From the hostel, take the U3 U-Bahn line from Westbahnhof (direction 'Simmering') to Volkstheater. Change to tram 1 (direction 'Prater Hauptallee') and get off at Hetzgasse. Takes about 30 minutes.