A motion activated sprinkler could work too. Or buy a motion detector and have it activate plant water sprayers (the kind used for automatic watering of potted plants).
https://www.bunnings.co.nz/pope-25mm-solenoid-valve-with-flo...
You can get attachments that allow all sorts of jets, drips and spray patterns.
It would probably plug in nicely in place of the water gun. However drilling through the wall and messing with the plumbing isn’t for everyone.