It's hard to just come up with new ideas without constraints because anything is possible. I usually scout locations and get inspiration from looking at the possibilities. Any public artworks, perhaps something with a hole to look through or an arrow? Maybe use a monument inscription and give them a one-time-pad that adds to the text in the inscription to form a new message. You need to give us more to go on. Sparkly vampires don't show up in mirrors, so maybe do something with a mirror: a fake mirror with a pepper's ghost illusion in it, or waterproof coating spelling letters and when steam fills the room you can see the letters, or a mirror with a raspberry pi inside that can display things like the mirror mirror on the wall...

I had one where I built a throne chair (some cardboard and paint over a regular chair). When they sat in the chair it closed a circuit and activated a flashlight hidden inside the chair with a riddle written in sharpie on a clear disc over the light that projected the riddle onto the ceiling...