It depends what you want to do with this new skill. Do you just want to tinker for your own pleasure or do you want to join with others to make music?
If you learn the ukelele, mandolin or guitar, you can make fairly rapid progress in producing chords for you and others to sing along too. But be warned, your fingers will get sore to start with before they harden up! If you want to be able to play tunes, then this will be a longer and much harder task.
The piano is a huge challenge to learn from scratch when you are older. Not only do you have to learn to read the treble and bass clef, but the essence of good piano playing is being able to find the notes without looking down at the piano - this is a skill that is best learned young when the distances become internalised easily.
And as for the violin - this might result in divorce from OH or neighbours! - it is one of those instruments that is only tolerable (or indeed wonderful) when played well.
The harmonica is relatively easy and there are lots of online courses and also books. It fits in your pocket and you can take it anywhere with you. You need to research this a bit as there are different types - some you can play in different keys; and some that you have a different instrument for each key. They are fairly cheap to buy. The only problem I have with mine is that I get out of breath!
I think a wind instrument would be another good option - only one clef of music to learn and satisfying tunes arrive quickly. Reed instruments like the oboe and clarinet require an initial struggle to get a note at all, and the flute requires a lip technique which you either get quickly or have to work hard at. You also need fairly flexible non-arthritic fingers that can stretch.
I really do think that the descant recorder is the best option. It is easy to learn, cheap to buy, and there are lots of recorder tutor books to be found. You will make rapid progress and maybe you could play alongside your GC, if you have any. It is a serious instrument and there are many play-for-fun recorder groups around - often with U3A. You can make a start for under £10 as a simple plastic recorder can be had for as little as £2 plus say a fiver for a tutor book. As you progress you can invest in a nice wooden instrument.
I envy you this exciting journey - have fun!!