A simple quest that can lead to Geralt and Keira getting their rocks off.
The Witcher 3: A Favor for a Friend
This quest opens up after completing A Towerful of Mice if you agree to help Keira again.
Head on over to the area indicated and with your Witcher senses you'll find a cart with tracks. You should follow these, but it's probably best to scout around a little and kill the Water Hag and Drowners nearby so they don't bother you.
Search around the cart to find the items Keira needs (by looting everything) then follow the blood tracks. You'll find the merchant but there are Rotfiends eating him. Kill them all, then search his body for a letter.
Back at Keira's hut agree to have dinner with her and head on over to the meeting spot. If you follow the right dialogue lines here (it's pretty obvious) you will then follow a trail of clothes into the woods and eventually get some sexy time.
Whatever you do here, you complete the quest and open up For the Advancement of Learning.