Mine are pretty simple, but definately different in one major respect: Red Onions.
They've got great flavor, and work nicely in tuna sammiches.
Tuna, Miracle Whip or Mayo (after years of having an opinion on the two, I've stopped caring), fresh cracked pepper, and sliced black olives. If you have any big mediterranean deli olives, hold the black olives and slice these guys lengthwise and put on the sandwhich.
Finally, a slice of onion (for texture, chopped onion is already in the mix) and some kosher salt. Make sure it's all on two toasted pieces of something respectable.