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.

I have quite a bit of near misses, but they mostly range from my childhood to the end of the "stupidity age" (~ 20 years), so I feel they are not so bad, and in any case they are easy to revisit , but some still bug me. In chronological order:

Romania (and more): it was a long trip as a kid with my parents, so I cannot really complain. Still, I think we missed many interesting sights on the way, and my father's visiting style of "seeing passing by car" did not help a lot.

Graz: school exchange at 13/14. Too young, too stupid. And no real visit ever of the city was organised by our teachers.

Barcelona: like winterkjm, it seems I totally missed many spectacular Gaudì buildings on our school trip. But parc Güell and the Sagrada Familia (still without roof!) we did visit, along with the Espanyol stadium, because our accompaning teacher was very fond of football. And it did not help either that for some unknown reasons we were based at Premià de Mar or some similar resort. Besides being very distracted by other more earthly reasons (we were 17 years old teenagers at the sea side...).

Regensburg: walk in the city when I was guest of a German friend of mine in a nearby village, walk of which I don't remember anything but a fountain.

Røros: I actually count it (as my first conscious WHS, nonetheless) as a visit of the second and half kind, as @jonathanfr would put it. But still it was too unsatisfactory and the bad mood of my travel companion ruined my mood. And we did not visit the museum.

Hollókő and Pécs: it was a very long and early solo trip in Hungary from which I learnt lots of things, but oh boy could I have planned it better looking back now!!! X-D Hollókő I for some reasons overlooked even if I knew of its existence and its WHS status and instead preferred spending a couple of nights in Eger: why?! Anyway, I liked it. And Pécs, I was right there at the cathedral, I knew it, but probably I was too confused. So nothing. And this bugs me a lot, because I was aware of it!!! Why?? Now that I see, I have to untick Pécs, which I erroneously counted as visited.

And lastly, Genoa: many visits, but never to that exact part of the city or for those reasons. But I can amend it easily.

Probably the most glaring near-miss for me is the Architectural Work of le Corbusier. I was actually staying in Auteuil in Paris, where two of his buildings are located, when it was inscribed in 2016. By my reckoning I even walked down the street parallel to the one on which Maison Jeanneret stands. But neither building was open on the days we were there and I felt I couldn't justify dragging my wife and baby one additional block just to look at the outside of a building.

Oh, and depending how much of a stickler you are, on that same trip I also missed seeing the door that is Pont d'Arc.

The miss that concerns me most, though, is the Ancient Villages of Northern Syria. I did consider a tour out to see them in 2009 (two years before they were inscribed) but decided to have an additional day mooching around Aleppo instead. Unlike the first two, I'm not sure when I'll get the opportunity to make up for that miss...