Apologies for posting in English. My German is pretty lousy.

I'm just in Vienna today, leaving tomorrow, but I'd love to spend the day just chilling out and painting (Warmachine for preference).

Is there a store open today (Easter Sunday!) where I could get something to paint, that also has an area for painting?

Indeed. I ended up spending the day wandering town, and few shops were open indeed.
Ah well, Wien still has many of the world's best museums, and it wasn't as if I'd seen even half of them before.