Crystal hardcase and screen protectors

Carrying bag if you take it out with you on a travvel
Spare styluspens, i already lost mine, and there not easy to find, styluspens for N3DS
Wrist strap^, you never know, but i learned my kids to use a wrist strap when playing and it saved a fall many times. (I use Wii controller Wrist straps, there's easy to find everywhere)
My Kids systems have full decal stickers on them to protect from scratches and then a clear hardcase (I know hard cases are more to protect from scratshes then falls)