Piano is easiest... it's an instrument with a single linear layout. IIRC there are 12 notes in an octave. These are repeated up the keyboard in a linear fashion. If you learn the notes in one octave then you know all the notes one the keyboard since the pattern repeats.
Violin.... sure you could learn it but there are things that will make it much more difficult than piano. The violin is very hard on certain muscles and bones. It's something that should be started at a young age.
Also...it has no frets lol. Guitar is easier than violin. A guitar at least has frets to help you find notes
A recorder is pretty easy to learn and it's very small and cheap and you can play it anywhere.
The most relaxing instrument I ever played was a Kalimba.