My neighbor and his kids keep coming over to play in my yard. Any pranks I could pull on them?

Every so often my neighbor and his kids creep into my backyard and play on this small playset I have (it's my son's) with my dog. It's been going on for a while now, and I honestly don't care, it's just that it's sort of awkward when they play and stare in my windows. This is where you come in AskReddit! I want a good prank that I can play on them that might make them think twice before coming over again. I've never been a good prankster, just the guy who puts plastic wrap over toiler seats.

PS: I don't really know my neighbor that much, but we've been around each other enough to say that we're friends.

EDIT: Sorry, I forgot to specify. I need a prank that doesn't hurt anyone or is very serious, just a bit playful.