The point is still to end the game with the most money, but there's a twist: In addition to Community Chest and Chance cards, there are cheat cards. They encourage you do to things like steal money from the bank, take a hotel from another player's property and put it on your own, or move someone else's token on your turn. If you get away with it, you get a reward. If not, you go to jail AND wear a handcuff.
It should be more lively because everyone will have to be actively on the lookout for cheating, while also trying to get away with as much as they can themselves.
We all get dragged down after a while, don't we?
Although if that's the case, maybe they should try playing with different people.