I had to prioritise and I opted for Wachau, Schonbrunn and more time in Vienna.
I think that is a good idea. Graz rewarded a longer stay, after the first few hours I wasn't particularly impressed, but relaxing into it a little more really made it an enjoyable place to visit.
Wachau was a really nice day trip from Vienna, and the summer should hopefully make it even more enjoyable.
If you have even a passing interest in Modern architecture I would take up Hubert's suggestion
of visiting the Postsparkasse. And more importantly Cafe Sperl!