one day in Athens -what do you do?

Acropolis, change of guard at Parliament or if raining visit the incredible National Musem of Archaeology.

Indoors - the National Archaeology Museum is the best.

Outdoors - the Ancient Agora was my favorite. The RS City Walk - self-guided tour - is excellent.

If you are in the city on Sunday, the impressive changing of the guard starts about 10.45, so you'll want to be there a few minutes in advance to get a good spot to stand. Other than that, there's a change every hour 24/7 so you can see it any time. It starts about 5-10 minutes before the hour. Also on the half hour the guards exchange places with quite a bit of ceremony.

There's a good view of the Acropolis from the restaurant balcony of the Acropolis Museum. You do not have to buy a ticket for the museum, there is access free access to the restaurant and you are not required to buy anything (though prices are fairly reasonable), you can just walk out and admire the view and take photos, both in daylight and after dark.

Acropolis and National Archeological Museum (taxi between them).

The line for the Acropolis can be very long. Go early or at the end of the day. If it’s just too full, you could go to the Agora instead. The Acropolis is an icon you really shouldn’t miss, though. There are viewpoints all over (the Acropolis museum, and there’s a pedestrian street that goes around the bottom) so you will SEE it even if you don’t want to go up.

If you love museums, add the Acropolis Museum. If you don’t want any museums, go to the Acropolis and other ancient sights in the area. You can buy a shared entry ticket for all the sights.

I would go first thing to the Acropolis and then to the agora. You can see a lot of the ruins in one day. I would go to the top two floors of the Acropolis museum which is specific to the Acropolis . I would wander through the plaka district.

I would not try to go to the national archeological museum as it is in a different area. I would save it for a return trip.

And make sure you stay central so you can walk everywhere.

We were there for 2.5 days and missed things I would like to have seen-like the changing of the guards.

I agree with Mira. You don't want to miss either the Acropolis or the Archeological Museum. Pay for a taxi between the two.

You don't know? You're right on the perfect website for someone doing this on his own .... Rick Steves has wonderful FREE audio downloads to the Biggies in Athens. Just click on this part of the website https://www.ricksteves.com/europe/greece/athens and t where it says "LISTEN" click again. Scroll down & it says ATHENS AUDIO TOURS . there are 4: download them to your PHONE
• ACROPOLIS - a Step-by-step DIY explains the mighty rock & Parthenon as you walk
• Ancient AGORA (the big area, downhill from Acropolis, fabulous when you have explanation
• NAT ArCH. MUSEUM -- steers u to TOP 10 Dont-Miss THINGS; can do it in 2 hrs or less
• ATHENS CITY WALK - U don't have to do it all, just go thru it with a map, and follow it selectively

Google maps on phone don't show the BIG picture, just what's ahead as you walk. Get yourself a PAPER map of Central Athens TOO. My online favorite - http://www.orangesmile.com/destinations/img/athens-map-big.jpg -- (click & it gets Huuge) is not printable in this size ... but THIS version is: http://www.orangesmile.com/common/img_city_maps/athens-map-0.jpg Just put your printer page layout as HORIZONTAL -- it says 2 pp but just print 1 ... all that you lose is a bottom strip, unimportant. And guess what? stop in the first hotel you see, ask if they have a little fold-up map -- and they have This!

Another printable (set layout Horizontal) that's simplified, highlights tourist attractions, limited street IDs.
https://www.planetware.com/tourist-attractions-/athens-gr-ath-ath.htm#GR107

Athens deserves at least 3 days to fully experience. You should be able to see most of what people go to the city for. If you want a day trip then the nearby island of Aegina would be fine. If you just want to stay in Athens then check out https://aktis.app/en

If you only have one day, make the most of your time. Hire a local guide to take you to the most important sites. They can arrange the day to suit your timeline, then drop you at the airport when finished.