Give them a secret treasure room:

When we bought our house two years ago The Boy was not quite 2 years old. The room that was to be his had a storage room attached to it. Our roof pitch is really steep next to his room, so it forms a triangular room 7 feet by 12 feet. The door is about 2 feet by 4 feet.

The storage room, aka “The Secret Room” had an old linoleum floor, a light with a switch, some wood paneling and some exposed insulation. At the time it was certainly not fit for the kids to use. And we didn’t figure a 2-year-old needed an extra room, but we agreed it would make an awesome surprise for The Boy at some point. So the dresser was parked in front of the door and The Boy had no idea for over two years!

Click through; they outfit it for him while he’s at school, and then surprise him with a secret hideout connected to his own room.