When I first moved in to my new space It had a boring and cramped toilet, which I knew I had to upgrade.
So I've decided to use old cheap comics I had and color it up.
This is a really easy and quick instructable, recommended to make with a friend.
Some cheap comic books you can spare (I bought a few for ~$1 each)
Wallpaper adhesive (~$4)
I really like seeing it every time I go to the toilet, really improved the room...
Step 1: Making the Glue+cutouts
Put some wallpaper adhesive in a bowl (bigger than the one shown, this was for demonstration). You won't need a big amount - I'd say 1/4 of a small bag as pictured - and add water at the ratio stated on the bag (consider it as "light wallpaper").
Mix it in the bowl for a few minutes till it gets gooey and thicker.
Cut the comics as you like. I used several different ones and most cutouts were small squares with plenty of action and color.
Applying: place the paper where you want it and smear a very small amount of glue with a brush or a finger. It's better to place them in layers and with some overlap to make it stronger and nicer for the eye :)
Make sure that the handle is usable and that all of the outgoing connections are still reachable.
When done covering as you want, make sure to glue any unattached corners.
Wait a few days in which examine your work and think of any changes you'd want to make before the next step.
Step 2: Applying Varnish
As the toilet (and bathroom) is pretty humid and not always well ventilated, the paper could break apart from the tank and ruin the whole thing.
This is why we apply a thin coat of wooden varnish on the whole thing, to keep it from any damage.
I can testify on this holding for a long time (this is a pretty old project).
Any simple wood varnish would do, the one here is just for an example.
Step 3: Done, Use Your Toilet
As said, you can use the same process on other smooth plastic and wooden objects, as I did on my trash bin.