I know how you can get the Choa garden, but what other GBA links can you do? and how?
The rest of the GBA things are online only.
You can link it to Zelda: The Windwaker to do the Tingle Tuner, Zelda: Four Swords for multiplayer, Final Fantasy Crystal Chronicles for multiplayer, use it to transfer chao from Sonic Adventure 1 over to Sonic Adventure 2 and vice versa (I transferred an immortal ANGEL chao to SOnic Adventure 1 this way.) Lots of things to do with it.
Erm... I think he meant for PSO only...