When I get my favorite Potato Leek Soup at the restaurant we frequent, it's always garnished with two fresh chive stems, resting criss-crossed on top of the perfect mini-dollop of sour cream. Oh it's a great soup, regardless of the garnish--but my point is, the garnish takes it over the top with very little extra work involved. We all like as little "extra work" as possible, I realize.
However, with my simple tips below, you can quickly take your homemade soup to the next level. Here's how~

Try sprinkling on freshly minced herbs, whichever kind you used in cooking the soup.
For curried soups and Mexican-influenced soups, stir lime juice into sour cream or yogurt with a pinch of sugar; spoon onto soup just before serving.
A little grated ginger, finely chopped orange zest, or a dash of curry powder are also good in a cream topping for curry-based soups.
You can, of course, change the flavorings according to the kind of soup you've made; a spoonful of cream will also help tone down the fire in spicy soups.
Toasted bread, croutons, crackers, or crushed tortilla chips add a satisfying crunch.
A spoonful of salsa, or a few chopped tomatoes, bell peppers, scallions, or cucumbers add a cool, fresh taste to your soup.
A sprinkling of finely grated hard cheese such as Parmesan or Romano, adds lots of flavor but not much fat.