The treasure hunt is one of the most classic adventure scenarios. You really can't go wrong with a set up in which you leak the distant and exotic location of fabulous riches to the PCs. Motivating adventure by threatening to have evil overrun the civilised world is always hit and miss, whereas appealing to pure unadulterated greed is guaranteed to get characters moving as fast as they can.
If you need even more motivation, actually set it up as a race. If the PCs don't get the treasure soon, someone else will beat them to it. And you thought they moved fast before!
Grogan: There's all this dough, see? 350 Gs!
Russell: 350 gp? That's not very much. You can't even get a suit of plate armour for that.
Dingy: 350 grand, not gold pieces! That's $350,000!
Grogan: It's in Santa Rosita Park. Buried under a big dubble-ya!
Melville: "Big W"? What's that mean?
Dingy: It means it's a riddle quest. We find it, dig it up, and there'll be a note with another clue.
Melville: Tell us more! Are there any traps?! Guardian golems?
SFX: shake shake
Lennie: Leave him alone, he's dying!
GM: The man kicks the bucket, due to trauma, exacerbated by shaking.
Lennie: Well, that's that. Looks like that's all the clues we get.
Russell: Anyone got Speak With Dead?
Dingy: I bet the treasure is guarded by a demi-lich.