Tiny Chao Garden
Here you can play mini games to win prizes and Jewel eggs. The eggs are transferable to your game, but toys you buy will stay in the Tiny Chao Garden. Make a temporary Tiny Chao Garden by sending your chao on an adventure through the adapters in the Hero/Neutral/Dark Gardens. If you turn off your Gameboy Advance, you'll lose everything though. That's why i guess it's better have the permanent Tiny Chao Garden by buying a Sonic Advance game, since you can save it.
Sonic Advance
Well, there's Sonic Advance 1 & 2. You buy them from a real life store, you can get acess to the Tiny Chao Garden. If you have these, lucky you, since you have a permanent Tiny Chao Garden.