You could try searing them.
If you think they are too far-gone for that, I sometimes use them (seared first) in a tuna casserole. I don't do my tuna casserole with heavy creamy sauces, it tends to be a lighter style of dish and the fresh tuna tastes pretty decadent in it.
Another option would be to use the seared tuna in a dip. I've done that with leftover smoked tuna and it was great.