My son is obsessed with Mario LEGO and had his own little table in the corner of the LEGO room. It was overflowing and my wife hated how messy it looked. While she was away for a few days I made this.
Two partial sheets of 3/4" birch plywood with French cleats attached, painted white, were mounted on the walls. Then the desk just slots into the second-lowest cleat, using the bottom cleat as support.
As he grows we can raise the desk a couple times, eventually bringing it up to 29" from the ground which is about regular desk height.
I also made a few shelves and a box for him to organize with out of the plywood scrap.
He likes that he can rearrange his storage as he pleases. I made a couple more medium-sized shelves that aren't pictured, and I may someday make a corner triangular shelf for Bowser to sit on.
That was quite well-executed.