There are three secret quests in Jorvik. You can only solve them all when you are Star Rider and able to enter Golden Hills. Here you find the locations and how to solve the quests.
Quest 1 - "Message in a bottle"
The first secret quest is hidden in a bottle, swimming in the sea. It's right before the Thorny Banks and between two rocks.
There's a letter in the bottle....
To figure out who should get the letter, you need to find the tree in Hollow Woods that has a carving on it. Follow the red lines on the picture to find the tree, because it is at the edge of the forest.
The carving reads: "Landon + Anna - 1972". Ride to Landon and talk to him. He will write a letter, too and wants you to bring it to James. You need to find a good place to send the message in the bottle back to Anna.
James found a place but the plan doesn't work. Now you can talk to James again and search for gunpowder. He wants you to shoot the bottle with one of the cannons.
Quest 2 - "A package by the side of the road"
This quest is not even hidden. Just follow the road to Firgrove. There's a package close to the fence.
Ride to Derek with the package. He'll have a closer look at the adress and tells you, that all he can make out is "F....y". There's just one person in Jorvik who can be meant: Felicity in Firgrove!
Quest 3 - "A mysterious dirt pile in the forest"
There's a mysterious dirt pile in Goldenleaf Woods!
Simply follow the road into the wood and go straight ahead to find the dirt pile between three small mounds.
You need to borrow a shovel from Jasper and dig up the hole in Goldenleaf Woods. You'll find a chest! To open it, you need to borrow a crowbar from Bob & Rob in the Smuggler's Cave.