As others have mentioned, there are many ways to go with this, depending on whether you prefer to focus on the lighter or darker side of the fey, as well as whether you prefer something tangible, a service, or something we would normally consider intangible.
Here are some suggestions:
Tears of a maiden over a love lost before its full bloom
The tooth of a monster never defeated by mortal hands
A dragon's remorse
A child's happiness
The child with the comet birthmark upon its shoulder
The hand of one born under Caiphon (interestingly could resolve itself via marriage).
A REALLY BIG favor.
A day of vacation (have the PCs run her kingdom for a day).
A proper husband for Mab's (currently) favorite niece.
A day of your lives (when they agree, have the PCs feel as though nothing has happened, but discover that a day has passed. Later have them run into people that remember what they did during the day they are missing).
An invitation to the Emperor's Ball.
They must be guests at her Ball that night.