KunstHaus Wien is another nice Hundertwasser building that you can actually get into (there is a fountain in the foyer that flows upwards (or something like that, that doesn't sound right...)). I love Hundertwasser :)
The Albertina is a nice gallery. Wander along Naschmarkt. Get a giant schnitzel from Figlmueller. Make sure you keep an eye out for Inspector Rex.
When are you going to Donaueschingen to see where your favourite river (disputedly) starts?
Not this trip...but have you read The Unlikely Voyage of Jack de Crow, by AJ Mackinnon who rowed/sailed a Mirror dinghy from Wales to the Black Sea?
Sounds like a book to add to my reading list.
I believe he used to be an outdoor ed teacher at Westminster - Tyson will know.
I'll add it to my list too :)