Want to give your Magic-using player a reason to use the Read Languages spell? How 'bout that third level thief being able to read that scroll? Well here are some great Ficitional Alphabets to write those riddles, clues and maps for your players. Alphabets from Atlantis, Tekumel, Tolkien, Star Trek and many more. Good Times!
And speaking of thieves and riddles in the dark here's a nice little link with some general Thieves Guild features & resources for DMs. It seems to be 3E based but there's plenty there to mine for your games from traps and tricks to riddles and more.
So I'll leave you all with this memorable little riddle of the Sphinx:
Round she is, yet flat as a board
Altar of the Lupine Lords.
Jewel on black velvet, pearl in the sea
Unchanged but e'er changing, eternally.