Wien to Graz; Stria & Carinthia

In Austria and a little out of your way, but Hohentauern and Murau and the drive between are nice. You could go to the lakes in Kärnten too.

https://en.wikipedia.org/wiki/Riegersburg_Castle
https://www.zotter.at/en/zotter-experience-world

Graz is a lovely town where you could easily spend 2 days.
https://www.graztourismus.at/en/sightseeing-culture/sights
Riegersburg castle and Zotter Chocolate experience have been mentioned already.
Additionally, I would strongly recommend to experience the Southern Stryrian wine road by car or by bicycle.
https://www.steiermark.com/en/Suedsteiermark/Region/wine-theme-trails/southern-styrian-wine-trail
There are several thermal spas around Graz like Bad Gleichenberg, Bad Loipersdorf, Bad Blumau, Bad Waltersdorf. Bad Blumau stands out by its architecture (by Friedensreich Hundertwasser)
https://www.steiermark.com/en/Styria/Wellness-health/Thermal-spas-wellness-resorts
About 1,5 hours north of Graz you could visit the Erzberg, the largest open-pit mine in Central Europe if that is of interest for you.
https://www.abenteuer-erzberg.at/en

For me Carinthia is an outdoor sports destination for any kind of activities in the mountains and on/in the water of beautiful lakes. Highlight is the Grossglockner high alpine road from Heiligenblut/Carinthia to Kaprun/Salzburg. But that is far out of reach for a day trip from Graz.
If you are willing to spend 4-5 hours in a car you could drive from Graz to Klagenfurt, loop around lake Wörthersee (https://woerthersee.360-tour.at/) , visit Pyramidenkogel (https://www.pyramidenkogel.info/en/), continue to Burg Hochosterwitz (https://www.burg-hochosterwitz.com/en/) and drive back to Graz.
Better option would be to find (maybe a lake-side) accommodation around lake Wörthersee for 2-3 nights and explore Carinthia from there.

Paul636
Your last paragraph covers my fondest part of Austria. My Grandfather lived on the Wörthersee in his later years, and we enjoyed the area when we visited.