Funny thing is I'm just building such a thing myself.
I've put an insulated lean too about 5x12m onto the side of my Workshop
putting the floor down next weekend.
First thing is Heating and Power..and Fridge
..and I've got a used Woodburner going in with a Hot Plate.
I should imagine plenty of Fishing gear...work bench..all me boating stuff (surprising how it mounts up!) couple of good Chairs and a Settee...my Large Hacker Radio..all the Prints and bits the Mrs doesn't like,some good Books!..and some good Locks! Inside and out!