This page of the guide to Dying Light 2 contains information about the hoverboard easter egg. You will learn how to get to it and what you'll get by finding it.
The whole quest begins in Saint Paul Island, in the church on the eastern side of the map. It's not marked in any way, you just have to go there yourself and explore the area. The screenshot above shows the church you must go to.
Your task will be to connect the cables. It is not a difficult thing to do. First, however, you need to find the cable's location. It's at the bottom of the tower, marked with a green number 1. You can get there by jumping from the top of the tower or by entering the church and moving the wooden board.
You have to take the cable and climb back up with it. To do this, use all the elements marked in yellow - the protagonist can grab primarily onto those. The cable will not run out unless it catches on something. Then you have to jump down again to straighten the cable and go back up.
Upstairs, in the bell tower, you will find the second part of the mechanism. Plug in the cable and the power will return to the tower. This will turn on the radio which you need to move on with this task.
Now that you've connected the cables, it's time to go even higher. Go out the window and use the grappling hook or the yellow elements that will allow you to enter further. You need to get to the highest room in the entire tower.
There, you will find a radio. Use it and hear what they have to say - start collecting the hoverboards. The first one is right next to the window. It is very colorful, so you will see it immediately. Once you got this board, jump out of the church.
Outside, you'll find some red traces that you must follow. It's quite a long way to go, but you can't get to the designated location right away, because you have to collect the rest of the hoverboards along the way.
You will be guided by the red tracks that will light up as soon as you use your survivor sense. Hold R3 to activate it on PlayStation, and the Q key on PC. You can find all the controls for all platforms on a separate page of our guide.
Eventually, you will reach the blue car, the trunk of which you can open. This way you will reach the last hoverboard.
You can find the Hoverboard Challenge in the southwestern part of Muddy Grounds. It does not appear on the map, although you can come back to it and repeat it until you succeed (or longer, if you like this type of gameplay).
The challenge itself is similar to parkour . Your goal is to ride/run into the blue circles. Part of the race happens on the water, the rest on dry ground.
It is worth noting that the hoverboard likes to get stuck and buggy. To alleviate that, set it up in the inventory again before starting the challenge. If you don't, it may be permanently "pinned" to the protagonist. You will notice this when Aiden tries to slide on the board instead of running on the sidewalk (which won't be very effective).
You will receive the appropriate amount of parkour points for completing the challenge.