Focaccia always has something on it. The simplest topping is a sprinkling of olive oil and sale grosso, coarse grained sea salt (Kosher salt is quite similar to sale grosso), but you can do more. For example:
Finely slice a few onions, separate the rings, and sprinkle them over the focaccia. Here Margherita has also added funghi sott'olio, mushrooms packed in oil.
You could dot the focaccia with cubes of pancetta, and Margherita did with another focaccia that will be pictured shortly, adding onion rings as well.
You could dot the focaccia with pitted black olives, or thinly sliced grilled bell peppers.
If you want to be simple, rosemary needles, or crumbled walnuts (or both, but that's not so simple)
Or add anything else that suits your fancy. The one thing I would be careful about topping a focaccia with is cheese, especially if you're using a hot oven, because if it overbrowns it will become bitter.
When you have finished sprinkling toppings, drizzle some olive oil over the focaccia, and sale grosso (kosher salt) to taste.