My refrigerator door is full of things that can add flavor to the most mundane ingredients. In fact, between the fridge and various cupboards and shelves, there are many different ingredients standing ready.
Although, like spices, some of them are expensive per pound, you don't use them by the pound. You use them in squits and dollops. So think of their expense per meal. And think of the flavor they can add to yet another boneless chicken breast or broccoli crown.
Here of some of my standards, all readily available in the supermarket. (Hint: different grocery stores in the same chain carry different things by neighborhood. Use this to your advantage in the hunt for flavors.)
Fresh on hand? Different things different weeks but always one or another thrown in the grocery cart. Garlic, shallots, onions, lemons, parsleys and other herbs. Peruse the fruit and vegetable section and ponder what could be used for flavor. Think of a Granny Smith apple as a flavoring agent. Or a single jalapeno. Is the dill fresh this week?
Barbecue sauces are ever at the ready to perk up a pork chop, moisten some brisket, or give kick to pulled pork.
Sesame oil, fish sauce (Nam Pla), soy sauces of various strengths, and other Asian ingredients.
Jarred pestos and tapenades, e.g. the classic basil pesto and the more recently available sun-dried tomato pesto, black olive tapenade.
Vinegars: white, cider, wine (red, white and rice), and balsamic.
Salad dressings for quick marinades of fish or chicken breasts.
Various tubes of goodies like black olive paste, sundried tomato paste, and pre-chopped dill.
Wine and other liquors. Since I'm cooking for only two, I find the little four-packs of wine to be convenient for cooking. Want to make a spectacular dessert that calls for Grand Marnier? Know what liquor store in town has the best mini-bottle collection.
Of course, I don't have all ingredients on hand at all times. Who has the money or storage for that? But by picking up a few things weekly, the larder is always ready for some experimentation.