These won’t necessarily decrease calories but can make a lighter meal, increase flavour and in some cases increase protein and decrease carbs.

My top 10 food swaps:

Plain yogurt for sour cream. I realize this is old news but it is great news. Use a greek yogurt to get the same tangy flavour while adding more protein to your meal.
Applesauce for fat. In your next batch of muffins or cake replace your fat called for 1:1 or use half the amount. (Recipe calls for 1 cup of oil, your choices are endless. Use some applesauce (or any fruit puree) and some oil or totally replace the oil with applesauce.