D&D: The Next Generation: Recommendations?

My son Marty has been pestering me to run a D&D campaign for him and his friends this fall. I’m going to try 4E, mostly so I can check out the new material in full. I have the basic books, of course. Can anyone recommend other must-have bits for the game? I’m heading to Gen Con this week, so I’m looking to pick them up there. Thanks!