I use Laughing Cow cheese wedges (between 35 and 50 calories) in place of both mayonnaise and cheese on my sandwich. I don't like mustard. And I can't eat dry sandwiches. The wedges are available in a variety of flavors:
Creamy Swiss, Light Creamy Swiss, Sundried Tomatoes and Basil, Garlic & Herb, and more.
This is what they look like: