This riddle was engraved on a chest in the strongroom of a bandit lair that my players infiltrated in Session 7 of my campaign.
Each of the symbols on the brass plate were actually buttons that the players had to press to open the chest. Press the correct symbol and the chest opens. Choose poorly and it triggers an electrical trap. You can easily scale the DC for the trap to the level of the players using the traps section of the DMG.
In my game, it took the players a bit to figure out that the symbols were buttons, and then they initially thought the answer would be a combination of several buttons.
The rogue started poking blindly at the symbols using a 10-foot pole. I was nice and gave her a circumstantial bonus to her reflex saves to avoid the inevitable zaps of electricity.
Eventually, some of my more thoughtful players figured out the answer.
Give up!? solution