A relative of mine once free hand painted murals on a structure such as this with exterior paint she happened to have around the house. It took her ages but she kept working at - I think there were various scenes on different walls: an apple tree with a bench under it, some vines up it, a garden walk with fanciful colorful flowers, perhaps a bridge with a stream under it.. perhaps a light house. I am not an artist - but you can achieve amazing results with brush strokes and color if you thing about it a bit first. You can put circles together and make flowers. straight brush strokes can be leaves and stalks, dabbing can become a full tree, etc.

However just in case (LOL) that should be too much work - how about some old window frames painted a different color from the walls and hung on the wall with shutters and a window box. Paint the door a different color from the walls and match that color to the drain pipe,

Or treat it like a small house and put s a few small shrubs around it, color it up and make it look cute.

Or combine all that and paint a window and curtains on one or more of the walls - all straight lines but fun colors. Also a welcome sign by the door or a hanging.

Actually this site has a collection of sheds that will give you lots of ideas...