For the month of March only, the Palais Liechtenstein, notable for its library and art collection, is open and free to the public. The palais is usually only open to those registered for a special guided tour, which are nearly all in German.
I visited yesterday and was greeted by an amazing golden carriage from the 18th century in the main hall. The library is open as are several rooms of art. Important Canaletto's are a part of this special exhibit. I will mention that that only the first floor is open and free in March.
On a warm/sunny day, the gardens surrounding the palais are lovely as are the nearby Strudlhofstiege (an outdoor Art Nouveau stairway). The palais is located in the 9th District, just north of the Ring. On your way back to the Ring, check out the Servitengasse between Grünentorgasse and Porzellangasse for some cute shops and cafes.
Palais Liechtenstein link - https://www.palaisliechtenstein.com/en/visits/the-constant-prince.html