I haven't drawn or designed anything for mine myself but I've added a skin to my silver 3DS XL then put a black semi transparent glitter hard case over the outside of it, my Yoshi 3DS XL I've left alone and I have Monster Hunter and Zelda faceplates for my N3DS which I alternate. I've added cute charms and jacks to all three and they have cases.
I've had a hard time deciding what I want to decorate my 3ds with. I was worried I'd mess up more elaborate vinyl placement, so I kept it simple by adding a Puddle Bunny sticker. It sits inside under the left speaker which is a good place because it won't get worn out or rub off.