On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the mission: A Gift from the Past.
Hyrrokin tasks Eivor (Havi) to find a gift for Gunlodr.
Reward: 1,700 XP.
Find Thrym's house
Go to the place marked on the world map. Next to the ice hole at the ice rink, you will encounter an elite enemy accompanied by two shapeshifters. Defeat them, then jump into the ice hole.
Once you're underwater, swim into the doorway marked in the image above and keep swimming ahead. Once out on the hard ground, break the ice wall and proceed to this location's remainder.
Search for a gift
Jump into the water, turn to the left and swim into the frozen corridor. Jump up to a higher level and smash another ice wall.
On the piece of wood, you will find a suitable item - Thor's bridal circlet. The mission will end and you will receive 1 700 XP as a reward.