They're not interchangeable. 'leker' is something you do for fun, 'spiller' is something you do when you have some rules.
You 'spiller' fotball, but you 'leker' with the ball.
That's like dialectal English "to laik", which means to play or mess around.
Thats what Bruce (my dog) and Mushy (my cat) do all the time! Bruce og Mushy leker!
We use "spiller" for that.
Using "leker (seg) med" in relation to an instrument would either mean that they're using it as a toy rather than an instrument, or that they were having fun with the music and just "playing around" on the instrument.
My God, so many new verbs in this lesson!! How am I supposed to remember all of them??!)))