I have a BestSkinsEver clear coating, Moshi Clearguard keyboard cover, Speck SeeTHRU Satin (black) hardcase, and an Incase 13" Nylon Sleeve Plus. Excessive, I know.
If you want to just protect it against scratches, perhaps get a BestSkinsEver/Invisible shield coating set ($25-$50, can be a pain to put on if you don't have steady hands) and any soft sleeve or bag with built-in sleeve. If you want some minor dent protection you can get a hardcase like the Spec Seethru or Incase hardshell case (be careful though if there is a little dust between the case and the Macbook it can actually cause scratches!). Be aware that the hardshell cases do add a decent amount of bulk and weight to the laptop.