My son is a big fan of LEGO's. I have a large bookcase in his room. He has the boxes stacked up on the bottom shelves and he lines the upper shelves with his completed kits. He is very adamant about keeping the box and instructions. He is particularly fond of the various planes. He lines them up diagonally by size. It looks nice.
I've been trying to come up with additional display ideas myself. His collection is growing! :)