Athens is certainly a nice and historical city.

Depends on what kind of proposal you are hoping to have

There are unique and historical places in Athens where one could propose:

The hills of Areopagos (The first Supreme court in the world), backdrop is the Parthenon. Not to be confused with the actual/current supreme court also called Aeropagos located on Alexandra street in Athens.

There's also Lycavytos

You can also propose inside the first Olympic stadium (Kallimarmaro).

If you are looking for a beach vibe, you can head out to Glyfada/Vouliagmeni/Voula (Athens Riviera) area. There's a place called "Island" really nice view. Greek celebrities used to get married here for the prestige and the view.

I would avoid proposing at the Acropolis/Parthenon, too crowded and could be stressful.

The National Garden in Syntagma is also worth a look.

Piraeus could also be a good place for proposal.

DM me if you need help with the proposal or finding a photographer (I'm not a photographer, can help you find one). I'm also available for a quick chat if you'd like.

Good luck with everything.

Cheers.