I have a few suggestions:
low cal/fat mayo.... doesn't taste quite as good but does the job
lemon and ground pepper
Not sure where you are and if this is an international food, but extra light philly spread, it is a type of cream cheese spread but not at all like the heavy cheesy version. I use it for all sorts of things. Here in Australia it come in 3 different fat versions.
I also use flavoured tuna cans, some are very low fat but some are not! read the labels.