Piano is way up there, not just because it’s mechanically simple to learn and understand (press button, note plays, always same note in same place) but because there are just massive amounts of resources to learn how to play. When I had more time, I had a lot of fun playing songs on a light up keyboard hooked up to a computer using Synthesia. I would download a song I wanted to learn (eg Moonlight Sonata), and the keys I needed to press would light up, so I would play the song bit by bit, building muscle memory. I was able to use a music sheet just to remind me of what my fingers already knew. Is it the “right way” to learn? Probably not but it’s fun and easy and that’s what I wanted.