One thing that's always in books I write are meta references to things such as fictional (mythical?) rock bands, restaurants, clubs, and such.
It amuses me to create such places as part of my worldbuilding. I put them in stories and my partner makes them into designs that can be marketed as merch.
Some examples include:
THE WOLFSHADOW TRILOGY
LupitolCafe AmaranthCHOSEN
Mercy River Whiskey FlaskTHE THING IN YELLOW
Pallid Mask TeeTHE SHUTTERCLIQUE
There'll be a lot of meta-merch for the SHUTTERCLIQUE series, but here are a few starters...
SHUTTERCLIQUE Merch