On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the Stray Naps side quest.
Rewards: 1,200 XP.
In the place marked in the above picture, you will meet a sleeping Viking man. Read the note the man is carrying. There you will find a request to bring it back to Grimsby, where his wife is.
If you want to fulfill the sleeping Viking's request, put a man on the rump of your horse and ride to Grimsby. While you are there, you will hear a woman calling out to you and asking you to bring her husband into the house. Put it on the ground and talk to the sleeping Viking's wife to complete this task.