Set a clear rule about basic kindness, then moderate anything mean.

Made that mistake once, never again.

Did a friend of mine once build a whole closet just for his board game collection? He said it was to keep everything sorted (which it did, honestly), but we all knew it was just an excuse to show off his fancy woodworking skills. The shelves were so nice that we ended up playing games there more often just to look at them. Sometimes the project itself becomes the main point, not just the stuff you put on it. Now I see any custom shelf and wonder what the real story is.