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)
- Vinegar
- Chili Powder
- Paprika
- Hot sauce
- Chimichurri
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.