How about a motion sensor that turns on a strobe light? Random things can look like they are moving and scary.

There should be some way to rig a toy that has is motion activated. The baby songs can be replaced with airplane noises, or truck noises from other toys?

You could also download sound affects, and play them with an old phone or Bluetooth maybe get Alexa involved... Alexa play a vacuum noise at 2am every night at low volume.

Lol I'm having fun with this cat scaring project

Very good point lol.

You need to sand off the shine before you put anything on it, otherwise paint will peel and stain won't get in. If you want it to look wood-ish, use stain. The easiest/cheapest way would be paint. Sand it JUST enough to get the shine off, use fine grade, so you don't have noticable lines. Paint does best with a few light coats that dry in between... 1st coat will not really cover much. It's ok. Let it dry, do it again. If you mess up, sand that spot a little, and do it again. It's up to you, if you have the area to do spray paint, cover the chair with a trash bag and go outside .. not next to cars or anything, and not if it's at all windy, cz dirt will get in paint.