there are tons of aftermarket mods for the Mazda 3...bodykits, exhaust, headers, and more...
example http://www.extremedimensions.com/shopdisplayproducts.asp?SID=4Mazda&id=392&cat=3+HB+04%2D07&ProdType=Body%20Kits
changing the stock stereo is easy...
good luck on modding your new 3