A Favor for a Friend - Velen
Location: Keira Metz's house
Quest Giver: Keira Metz
Other Requirements: You can pick up this quest after completing A Towerful of Mice
Accepting this quest will auto fail the Lynch Mob Quest, if you have not already done it. The quest is a very short and easy one that takes place at Hangman's Alley, the same place the start of this quest does. Additionally this quest allows you to romance Keira at the end of it, your choice to romance her or not will have no effect on the story of future quests.
For this quest once you're at Hangman's Alley use your Witcher Senses to follow the wagon's marks off the road to the southwest. Collect Keira's package underneathe the wagon and (if you want) head over to the Rotfiend nearby and slay it, next to where you found it there will be a corpse up against a tree.
Inspect it for a letter which will give you a bit of back story. Once you're done here return to Keira's hut and speak to her to advance the quest forward. Agree to wait outside and agree to be her prince charming for the night to sleep with her - the rest of the dialogue options don't really matter for the quest. Nor does winning the horse race against her.
You'll spend the night with Keira on the beach and gaze up at the stars regardless of whether or not you chose to sleep with her. During your discussion with her she'll put you to sleep with a spell and when you awaken the following morning she will be gone and you'll be given the next quest in the chain, For the Advancement of Learning.