I strongly recommend the
ALS Nintendo DS Traveler Case, with "play thru" -- they are everywhere (Target, Circuit, BestBuy, Kmart, etc.). They usually cost $9.99, some of them have Mario on the front of them, they used to have Pokmons Dialga or Palkia. They have a wrist strap, carbiner (clip), and a zippered pocket on the back which will hold some games and an extra stylus or two. They have a clear plastic sleeve inside into which you slide the DS, so u can play with your DS while it is in the case.
We have 3 in my house, though my youngest (7) prefers not to use the case (thinking about getting him a Nerf case), but they seem pretty durable. One I bought @ Target had a defect with the zipper (returned it for exchange), but going on 6 months they are still pretty durable. My only complaints is that you can't get the stylus out of your DS once it is in the sleeve (so keep a stylus or two in the zipper pocket), and the zipper pocket, while it holds a lot, is a little tight to dig around in with adult size fingers. Two thumbs up otherwise.
http://www.circuitcity.com/ssm/Nint...sem/rpsm/oid/200330/rpem/ccd/productDetail.do
http://www.circuitcity.com/ssm/Trav...sem/rpsm/oid/181741/rpem/ccd/productDetail.do
Be sure to pick up the Hori Screen Protectors too -- they are the best in terms of quality and a no-smudge adhesive -- there are several threads on CAG about them, along with the Hori knock-offs too. The "Hori" brand logo appears in the upper right corner, and you can get them at Target for $9ish. Worth the investment to protect the life of your DS.