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!

I suggest to have a system, a scale, which determines the degree to which we visited a World Heritage. We could learn from the movie "Close Encounters of the Third Kind" from UFO-ologist J. Allen Hynek's classification of close encounters with aliens:

- First kind: being in the central area without touching the building
- Second kind: staying on the outside but having touched
- Third kind: being inside the building, having touched or not

There is also the fact of seeing the building being far outside the buffer zone, or being in the buffer zone without entering the central area.
We can also think about how to look at things with the property without special buildings (natural sites, cultural landscapes ...).
All this to launch the debate on how to validate or not a site visit. And how to see things for serial properties: having "seen" a single well in the series it is enough to say that we visited this World Heritage property? It would harmonize things so that we can all have the same criteria for our account visited property.

One big frustrating one for me:

Bat, Al-Khutm and Al-Ayn (Oman) I drove around the desert in Oman for hours trying to find it. There were no signs. I could find nothing online giving instructions for how to find it.