Guess the weight.
Player will guess the weight of a random item presented by the NPC host.The items can be anything from loots, equips, consumables, etc.
Player with the closest guess-timate (which does not exceed the item's weight) gets a reward Player who can guess the exact weight gets double rewards.
Have a 3-second countdown to prevent players from just googling the answer.
In case players have the same guess, the first to have given his answer will be the winner.
I'm no coder or what, so i don't have a clue on how this will get implemented. Maybe players will give their answers to the NPC by typing in the value like how the coin exchange works or how shops work, i don't really know.
But as always, feel free to leave a comment.