Going to be an organ donor. What can I donate if my family wants to have an open casket in the future?

Almost everything. Nearly everything can be safely donated without compromising an "open casket" funeral. This should be discussed between the organ procurement counselors and the family before proceeding with donation.
Most organs. With the possible exception of a rare face transplant, virtually all other organs and tissues can be donated, and still have an open casket funeral. I have been told that face donors have a mold replacing the face at the time of donation, but that may be a personal or family decision at the time.
All organs - thanks! Organ donation is accomplished through an incision in the abdomen and chest. There is no problem with an open casket. Thank you for the generosity of your thought for others.