For me there are a few.
I love watching tennis and wish I had taken lessons as a child and worked my way up to compete with the best.
I wish I had learned keyboards, the piano and how to read music. I would have been in bands and have been a session musician and even composed music as a profession.
I actually learned drums, in my teens, never well enough to be a top performer though.
I wish I could have been a professional comedy and joke writer.
What do you wish you had got into and learned when young?