To say I quite adore food is an understatement. The idea of the Garden of Eden with fruit and vegetables everywhere is heavenly, and fantasizing about the gardens has gotten me through more than one Sunday School lesson. The species of the forbidden fruit is a somewhat contentious topic, and I find it pretty intriguing. If it is interpreted as a literal piece of fruit, there's a wide range of possibilities.
The apple is the first thing that comes to mind for me. According to the great Wikipedia this is where the anatomical term "Adam's Apple" comes from, and it's most commonly used food in artistic representations. However, according to this article, and pretty much every other piece of research and speculation, there are quite a few different traditions.
One that stuck out to me is the Judaic tradition of wheat. According to the aforementioned mentioned article, "...wheat was originally meant to grow on a tree, not as a grain, but as bread already baked. After the sin, this tree which would grow ready-made baked goods was reduced to a lowly plant which had to be harvested and processed to produce flour."