GBA SP is really powerfull.
Last couple of days when I got spare minut, I play Advanced Wars (recomend this to all strategy funs, this game remainds me of BI, but much better and more advnaced gameplay).
Also Castelvania Cyrcle of the Moon is great game.
And yet the best of all platformers, Metroid - Zero Mission.
IMO GBA is what amiga would become if company did not close in 1995.