If you buy a gameboy at the H.M. Boutique, it will change into a gameboy Advance if you play Hamtaro on your own GBA.
Make Lots of Sunflower Seeds Fast
Go to the Ruins and at the very first map, go to the puddle of water near the flower. Dig-dig until the flower lets the seed out. Get the seed and leave. Come back after you exit, and do the same thing. You get 10 sunflower seeds each time, and can easily make hundreds. (This also works with Tack-Q Bowling. If you have all the Shogun Wigs, they give you 20 sunflower seeds.)