What's your gaming style? There are some great, great titles for the DS I can easily recommend, such as
Mario Kart
Animal Crossing
Phoenix Wright
Hotel Dusk
Rune Factory
Puzzle Quest
Meteos
for starters. Still, depends on what genre you like. If you like puzzle games at all, the DS is a great system for them, and Meteos is a really enjoyable experience that you could pick up quite cheaply at this point, although possibly online at online retailers or preowned at GameStops.