I usually use a little bit of butter, soy milk, mayo and dijon mustard or horseradish. You could leave out the butter and mayo and use chicken/veggie broth instead of soy milk. Dijon and horseradish adds some nice flavour. For the garlic, I've thrown whole cloves in the water with the potatoes and mashed them right in too. Another thing I tried is making half turnips and half potatoes. It is really yummy!
And I'm with Cassie, I say don't tell your FIL that you are making low fat mashed potatoes and just make them the way you want. I bet he won't even notice!