I use dijon mustard to add flavor all the time. I also use onions in almost everything I make. Fresh herbs are also a great flavor booster.
I make a lot of chicken breast which doesn't really have a strong flavor so I buy a few different bottled sauces to jazz it up. Most of them are low calorie and fat free just watch the sodium.
If you like spicy food you can also use hot sauce or pepper flakes. My husband loves spicy food so I marinade chicken for him in sriracha, fresh garlic, and a little olive oil and then just grill it or broil it.