What's the most beautiful place where you've ever played your instrument?

In the Palais Kinsky in Vienna. 

Sage Chapel, Cornell University

We visited our son who resides in Manhattan. On a cold very rainy afternoon I was able practice in the 65th story lounge, on a baby grand.complete with fireplace and floor to ceiling windows with a view of the world.

Probably in our home in Colorado but we don't live there anymore. I envision a concert at Cabrillo Monument [ Pt. Loma, San Diego]. I actually played for a proposal on the point [ outside]...against park rules but somehow we made it work. But in the gift shop [ after hours  would be spectacular]...I've looked into it and the cost would be the challenge:)

I have never played by a stream or on a mountain....BUT....THE MOST beautiful place.....I was working in Toronto and looking for a place to wash my clothes. I was carrying a huge load of dirty laundry in my arms...aimlessly wandering down Bloor Street looking for the hotel-recommended laundromat that apparently wasn't there. After walking a couple of miles, I passed a a piano store. I went inside and dropped my laundry on the floor and for solace started to play something at one of the pianos. A salesman came up....I explained to him the sad story of my wandering through the city with dirty clothes. He said...."We have a special piano in the back....if you want to play." I left the clothes at the front of the store as sat down at the piano in the back room. It was exquisite. The tone...the touch...I almost started crying. I went back to the sales person and said, "I will buy this piano. I don't care how much it is...please....I'll sell my house...give you anything..." He smiled and said, "It is beautiful. I know. It's the best we've ever had. But it's not for sale. It is Alfred Brendel's piano when he plays concerts in Toronto. There is nothing like it." Then he called me a cab with directions to the laundromat. The MOST beautiful day.

Sala São Paulo, Brazil.

I was with friends in the Lake District in England and we visited John Ruskin's house. He was a philosopher, artist and musician. His piano was in a room looking out over the beautiful lakes, and was open with some sheet music on the music stand. The music was composed by John Ruskin and I played it through twice. A wonderful experience!

The most beautiful placed I’ve played: Rosario Resort, Orcas Island, WA state - I played on the old Steinway (c. 1900) in the museum (the one behind the ropes in the picture in the link). And I was able to play the organ as well. 
https://rosarioresort.com/museum

I like the cover piano photo .

Spivey Hall near Atlanta, Georgia. One of the music groups I belong to has an annual members concert there, and I have played in it the last two years. A beautiful recital hall with two Hamburg Steinways to play. Of course, we have to pay to play there, but it is an amazing experience!

Every year the San Francisco Botanical Garden hosts Flower Piano. They place grand pianos throughout the park for anyone to play. It's so beautiful and peaceful to play among the gardens. 

Well... my main instrument is the organ, so there is an essential connection between the place and the instrument. First place would have to go to the Grote Kerk, Alkmaar; second place to St Gervais, Paris; also wonderful in their different ways were the smaller churches at Oosthuizen and Krewerd. In the UK: the cathedrals of Norwich, Hereford, and Worcester; Christ Church, Spitalfields (London); and Leominster Priory.

I don't know if this is the most beautiful place, but it was certainly the place with the most gilding!  This is one of Liberace's pianos, in Las Vegas. 

I played at Ehrbar Saal in Vienna on a beautiful exquisite Bechstein Grand- the hall, the sound, I will never forget that

okay then
i'm sure it's not exactly a nice place, but it was a very exciting thing. i was at the vaccine center during the lockdown. In a huge room where people were waiting a few minutes after the injection, a music store had left some instruments on display as advertising. at that time i was also starting to take my first steps with Pianote AND Tonebase :)

I sat down at the piano and played a few chords and a simple melody in C, something Einaudi-like to say (more I didn't know how to do yet). There were a lot of people sitting around waiting to be able to leave the room and come back, and I'm glad I eased some of the tension of that day. 

Have had lovely pianos in airbnbs during month long stays in Rome. Prague, and best if all in Budapest in an apartment that shares a wall with Liszt's last apartment which is now the Liszt Conservatory and Liszt museum. Also played at our hotel in Kenya.