Arugula is a dark leafy green with a peppery bite. Adding it to your salad greens will give your salad extra dimension and flavor with very few calories.
Balsamic vinegar (especially aged vinegar) has a great flavor that intesifies and sweetens when you reduce it. Bring a small pan of balsamic vinegar to a boil and then simmer until it thickens. When it coats the back of a spoon, it's done. It makes a great salad dressing or veggie dip and is extreme...
I recently bought an all-in-one apple corer/slicer. One simple push, and my apples are ready to eat. I find that if I have slices instead of a whole apple, I'm much more likely to snack on them (especially during work hours).
I've started keeping a bottle of lemon juice in my refrigerator and another in the fridge at work. I drink way more water that way! It's also great to have around so that I can cut up fruits and keep them from oxidizing.