I love teaching Piano. It's not the right instrument for everyone, but it's a fairly accessible instrument for people of all ages. It's wonderful for building grey matter and increasing your fine motor skills. Best of all though, it's fairly easy to see and understand music theory due to how it's laid out.
Piano is a gateway for learning how to play what you audiate in your head, learning any style of music, learning to read music, developing a great sense of time, and developing limb independence.