GBA games you can play forever?

I want a GBA/GBC/GB game that I can play forever, since I'll probably never buy another. All I have right now is Pokemon and Advance Wars - both of which have somewhat of an unlimited replayability.

There is also Harvest Moon, but it seems to be sold out everywhere. Puzzle games that become stale after a day not wanted.

Thanks.

You've already got Pokemon covered.  Emerald has the Battle Frontier for tons of extra things to do.  Seconding Tetris or your favorite puzzle game.  Puyo Pop Fever rocks.  Final Fantasy Tactics Advance isn't bad.  You'll be at it forever if you decide to perfect it or go with challenge games similar to the original FFT.