The other idea I have is to gift her this stuffed toy I saw which was available in different sizes. Then replace it with a bigger one every few days. This sounds like smth she'd quickly catch onto tho.

Create a creepy deja vu scenario spanning a few days, then act like you have no recollection of the first occurrence

Buy a skeleton and buckle it into the back passenger seat.

You can also sit them on the toilet.

The best one I ever pulled off was doing nothing.

One year my wife got me with a random prank a few days before Halloween. I told her that it's OK Halloween is coming, and I'll get my revenge.

It turned out that she spent the few days in between checking everything and everywhere in the house. Every time she opened a door she did it carefully, in case anything jumped out or fell on her, she wouldn't let me make food or drinks in case I did something to them.

By Halloween, she was terrified, but didn't know what to be scared of. Her car was parked in the garage, and it took her about ten minutes to go into the garage, check around the car, get in, then open the garage door.

The day after Halloween, she asked me what the prank was because she must have missed it.

I'd completely forgotten about getting revenge five minutes after I'd said it to her...

Option 3: Take random photos of her when we're out and send em to her from a random number acting like a stalker? Seems too far tbh