Personally I hope it's not a weapon, it's a casino after all and them keeping an overpowered weapon wouldnt make sense. Millions of caps would make sense but be a bit boring, so I hope if the treasure is riches it comes in the form of some valueble collectible items like gold statues etc, which will be worth alot, but traded in for unique items with the right people.
OR possibly some kind of pre-war tec, I can' really think of anything from the top of my head but maybe a machine that takes the radiation out of food or purifies water etc, which you could either give to the fraction of your choice, or keep to put in yoursafe house.
