A while back I developed this whole headcanon about how Simon taught Marceline how to play the guitar. I thought I was just sooo clever for having thought of it, but as it turns out, just about everyone else had the exact same idea. XD Well, I’m glad we’re all on the same page about this, at least.
Basically, my headcanon is this: For a couple of years when Simon was in school, he played in a band with his friends as a hobby. He was mainly the drummer, but he could also play the guitar as well as write songs. (That’s why Simon’s notes in ‘I Remember You’ rhymed; he originally wrote them as songs.)
At some point when he and Marceline were together, Simon found a guitar and, after she showed an interest in it, decided to teach her to play. (He was actually able to find two guitars; a normal-sized one, and a child-sized one.) He would also play it for her as a way to help her sleep when she had nightmares, and to help soothe his own increasingly distressed mind. The times where he gave Marceline lessons, or listened to her sing, or played games with here were what Simon treasured the most. These small, mundane activities allowed Simon to return to the sense of normalcy he so yearned for, and he could forget, just for a moment, the hell the two of them lived in. When Simon eventually left, Marceline continued to teach herself as a way to honor him.
(A small part of me has considered doing a comic about Simon and Marceline’s departure, but it’d be kinda long, and, to be quite honest, I’m pretty sure it would make everyone in the Simon fandom hate my stinking guts. So, I should probably not do that, huh?)