I'd keep a bottle of my favourite salad dressing in the fridge (labelled as mine). That's if you like salads, of course. Maybe you even could keep some fresh vegetables there, to make salads. Or at least a large bag of baby carrots for munching/snacking, especially if your coworkers bring free sweet goodies which you'd rather avoid.
If there is a freezer, perhaps you could store frozen foods that can be cooked using the microwave - either stuff you make at home in advance and then freeze, or bought items, like "ready to cook" chicken or something (just check the nutritional info and ingredients and make sure it's healthy and that you want to eat it).