Two ships on either side of eachother. Battleships.FIGHT
A large mansion with one invisible person that goes around killing everyone with a knife or sword. Everyone else gets a knife/sword thing as well and has to kill the invisible dude.
I'd do it myself but It takes me a lonnnggg time to build something.
Maybe instead of invisible, it would be more like TTT.
I've never played TTT though my friends have told me about it.
Challange, CityRPG, DM? idk
All of them are stuffty overused ideas.