What to do with 1 day in Vienna, June 23?

Well, if you were a WWII history buff, I'd recommend going out to Arenbergpark to see the Flak Towers (Flaktürme).
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And for just flat-out magnificence, how about The State Hall of the Austrian National Library @ Josefsplatz!
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And the endless beauty of the many fine Jugendstil façades throughout Vienna, so look-up when walking about.

And if you are an art fan I’d highly recommend a few hours at the beautiful Kunsthistorisches Museum followed by a pastry and coffee somewhere in the old town.

There are also great maps and walking tours of the downtown and ring road to download on the RS webpage.

https://www.ricksteves.com/watch-read-listen/audio/audio-tours/austria-germany

Take this itinerary and choose the sights according your interests.
Incidentially, I never have met anybody who was able to do this in three days as suggested.

https://www.wien.info/en/recommendations/vienna-in-three-days

Walk the pedestrians streets up Kartnerstrasse past the Opera House to St Stephen's Cathedral, then Graben street to St Peter's Church, and down Kohlmarkt street to the Hofburg Palace. Stop for sachar torte at a Cafe or go to the Palmenhaus for a treat and sit outside overlooking Burggarten. The Volksgarten rose garden is lovely. We also liked the Upper and Lower gardens at the grounds of the Belvedere Palace. An absolute highlight was going to Schonbrunn Palace and gardens with morning tickets, though you may not have time for that. You could go to a Mozart concert at one of the churches in the evening if you like. I think you will love Vienna!

Periscope - giant hunks of concrete are not what I’d recommend to folks with one day in Vienna, but if I did make that recommendation, then I’d suggest Haus des Meeres at least as that location gives some historical background plus you can actually go inside the structure.

My ideal day in Vienna:

1. Go to Schönbrunn early and walk to the Gloriette, exploring the gardens along the way.
2. Return to Karlsplatz on the U4 subway line
3. Admire the Opera House and stop in Gerstner for a snack and strudel
4. Walk up Kaerntnerstrasse to Stephansplatz
5. Visit the cathedral and take the elevator up to the tower for a view of the famous cathedral roof
6. Walk down the Graben and turn left on Kohlmarkt, walking towards the blue dome of the Hofburg
7. Take a slight detour to the left to visit the spectacular National Library - be sure to peek at the Lipizzaner horses in their stables across the street
8. Walk back towards and under the blue dome of the Hofburg, then go through the inner courtyard and out at Heldenplatz
9. Walk through the Volksgarten to smell the roses
10. Cross the street to see the Rathaus (city hall)
11. Walk counterclockwise on the Ring, passing Parliament and the Natural History, Kunsthistorisches Museums.
12. Go through the Burggarten to Palmenhaus for a drink
13. Have dinner at Gasthaus Poschl

Notice I said if you were a WWII history buff

And there's no finer place in the land to see Flak Towers like you see them, standing in Vienna - you can't erase history.

I'd take a pass on the Haus des Meeres, it has been cannibalized into a commercial structure - climbing wall on the outside, aquarium on the inside.

Thanks to everyone for the great list of ideas!!

Periscope - I live next to Arenbergpark and go there about every day. I think Augarten is also better.

PS - I did notice the “if.”

You're correct, I confused myself, AUGARTEN is the place to go. Hey best to visit both locations to see the 3-different styles. There is one more flak tower in Vienna, but it is inside a military complex - you can see the top of it if you're over by the Parliament Buildings, if I remember correctly.
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Here is a nice link for some great visuals of Vienna:

https://iangriffinphotography.com/tag/innere-stadt-street/

Periscope - you might enjoy Wotrubakirche, DonauCity Curch and Umspannwerk Favoriten. I fine these buildings fascinating.

I’m also on Instagram if you want to see some other examples of architecture in Vienna. I’m 3point14maker.

Spammy link above, don't click. Reported.

Hasta luego.