Unless you havent already gotten the games make sure you get the Phoenix Wright games. I'd probably say get the ScummVM DS port and get the old Broken Sword/Monkey Island games on there. Get a few emulators, SNES, Mega Drive/Genesis that sort of thing. Check around and see what good homebrew Applications and games there are too.
Cartridge for the DS which allows you to play Homebrew Games & Applications, along with Backed up copies of games. It's essentially a DS Cartidge which allows you to put a SD Memory Card inside and the DS can access the card. They're supposed to be really good I'm after one myself.