If I had a 3DS, I'd want to put a small decal on the front, something in pixels probably. But I only have a green/white 2DS, which I think looks lovely as it is...
I haven't decorated my 3DS and I don't think I ever will. But when I was younger I would print pictures and tape them to my DS. TBH it was kind of cool and I would switch the pictures out a lot but it damaged the paint a bit doing that. Nowadays I have to buy my own stuff so I don't really like to do anything that might damage it.
I put neon nail polish on my DS because me and my little brother have the same one and I didn't want us to get confused, so I painted a "B" on mine for my name (Bridget) and did some splatter paint
I have a Princess Peach sticker on my 3DS, and I also have a Riolu charm in the little charm hook (although it's missing an arm, I have to glue it back on eventually).
I used to have TBT Fair stickers on mine but they eventually fell off and getting rid of the last sticky stuff was hell and made a scratch so yeah, idk if I'll stick more to it now.
I have a Hello Kitty skin on my 3DS XL. It's a sticker one that I ordered from eBay. I'd ordered similar ones from the same seller for my DS and 3DS, but I feel like like this one doesn't stick so well on the rounded parts. It's cute enough, but I might remove it at some point because I'm a perfectionist. Lol.