I had two great riddles, but now I'm not sure I remember the 2nd correctly...
The first one is a mod on a classic: you have a scale and 12 coins, one of which is fake (weights differently from the others), and you only have 3 chances of weighting two groups of coins against each other. If you know that the fake is heavier (or lighter, it's the same), then the riddle is a piece of piss. If you don't know it, then it's challenging! But believe me, it can be solved. It took me half an hour or an hour or so to find the solution back then however, so my suggestion is not to throw this riddle in the middle of the session, but rather at the end of it (or tell the players they can come back later to solve it) so that the players can think about it before next session.
The second one... I barely remember it now, so this might be wrong... There are 5 pirates (or whatever you want) who have a treasure of 1000gp to split among them. They are all greedy first and bloodthirsty second (you can explain what this means or let the players figure out, but the meaning is that each of them will always make a choice that makes him earn more coins, and failing that (or if the money outcome is the same) they will always prefer seeing another pirate die rather than not). Of course they are also all clever and don't want to die. The five pirates have rolled some (real) dice to decide in which order to proceed, so the 1st pirate has to make a proposal on how to split the money, then everybody will vote, if the majority is in favor the deal is done, if the majority is against it, the 1st pirate is thrown to the sharks, and the ball goes to the 2nd pirate, and so on. What is the best proposal the 1st pirate should make? (you can link this riddle to the in-game scene by for instance having 5 dishes or boxes with counterweights, and the PCs have to put the right number of pebbles on them to unlock the mechanism... optionally a wrong answer may unlock a trap).
Edit: forgot to say that in the pirates riddle, in case of a tie e.g. 2 in favor / 2 against, the proposal is rejected (also IIRC, there might be 2 equivalent solutions)
