Have them try to capture a princess for ransom. There would be several stages, involving different types of play
1) Planning - How are they going to nab her? Give them a limited amount of time to plan. The Princess is attending a special event or something that ends soon. Perhaps a joust? - Lots of would be heroes around!
2) Execution - They need to get the princess without harming her, they also might want to avoid killing too many bystanders/guards (perhaps on the orders of their superior - who just wants revenge)
3)The Hostage - Once they have the princess they have to put up with her. You could play her as fiesty, demanding, rude, or maybe somewhat sympathetic (she might long for adventure and knows what her father would do to the kidnappers once they are caught).
4) Negotiations - The King's men attempt parley. Many things could be done here. Perhaps the negotiation team are capable adventurers who really plan to attack. Better yet, the negotiators want to comply with demands but during the discussion the Princess escapes and they can't find her. Now what do they do?
5) Get out alive - No matter the outcome, the characters have to complete the mission and survive. Perhaps the adventurers are hounding them. Perhaps they've had a change of heart and try to deposit the Princess back home w/o getting caught, or maybe they've completly botched the mission and have to report back to their boss with the bad news.
I'd run it with an overall light hearted mood. More Shrek than hack and slash.