I know people who bring back the Oetker cream/pudding/glaze mixes. You can get some Oetker products in the US, but the selection is limited in comparison to Germany. These mixes are cheap, light, and easy to pack.
German chocolate is available in the US, but sometimes hard to find and always pricey.
I usually bring back some German teas that I like (I'm partial to East Frisian tea).
I used to bring books back, but since I can order books with no shipping charge from globalbooks.de (but with no MwSt refund from this site, unlike Amazon.de, which deducts the tax but adds a shipping charge).