Some foods have no real (harmful) nutrition, so adding them is basically free flavor.
Spice is a recurring theme.
Some of my favorites:
- Garlic (my breath is obnoxious after lunch)
- Dijon or Brown Mustard
- Chili Peppers
- Black Pepper
- Many herbs (e.g. cilantro, mint)
- Chili Powder
- Hot sauce
These are also handy in making honest vegetarian and vegan meals, where honest means "not trying to taste like meat". Adding loads of butter and salt is just cheating.