Here are a few Sizzy headcanons that I love:
Simon reads Sizzy fanfic to pick up on the lines that work with Izzy
Clary and Simon made a huge plan of how Simon was going to propose to Izzy. However, on the day, Simon got too nervous and mucked it up. Izzy had to propose to him instead. But he likes to tell everyone how sauve and awesome he was.
Simon once tried to teach Izzy to how to play his favourite video game. After 5 mintues she had mastered the game and beat him twice.
Simon converted Izzy to the Star Wars fandom. The first date they went on after Simon got his memory back was to see the new Star Wars.
Comment if you have any other Sizzy headcanons, and please like!!! I hope you enjoyed these.