If you are interested in learning more than one musical instrument in your retirement, then I think that learning the piano is the best to learn as your first instrument. Clearly, not everyone would agree with me because we all have our own biases, including myself.
I think learning to play the piano first is the best instrument to learn first because producing a sound is really easy – all you have to do is press down a note. With woodwind and brass instruments, you have to blow a certain way, maybe on a precise angle and not blow too hard or too soft to produce a decent note. When it comes to string instruments, including guitar, it can actually hurt your fingers to press them onto the fretboard. Clearly, over time you can get used to it, but in the intial stages, it can hurt the fingertips.
One of the benefits of learning to play the piano first, is you can learn what notes make up chords by playing them. With modern, popular music, knowing what chords are what and what notes make up each chord is crucuial knowledge in order to be able to play them, or work out which notes should be played by which instrument. If you want to play classical or jazz music, then the piano music can give you a real snapshot of how these two genres are structured. Basically, learning to play the piano first will give you vital theory knowledge without really having to focus on the theory.
Granted, learning to play several notes at once can be daunting, but in some ways this is no different to other musical instruments, even the ones that you can only play one note at a time still need coordinating of both hands and sometimes even the mouth! Learning to read piano music can certainly be daunting but this is managed by starting off reading extremely simple music and then very gradually working your way up from there.
I hope you can see that because playing the notes on the piano is actually much easier than other instruments and giving you an understanding of theory of music, learning to play the piano is a natural first choice of instrument to learn.