While you could probably top a pretzel with almost anything, there are certain toppings and combinations that work best.
Coarse salt is always a favorite. You can find "pretzel salt" at many cooking and baking supply stores, as well as select gourmet markets. Substitute another coarse salt if you can't locate it, but stay away from fine table salt — the fine grain can melt into the pretzel as it bakes, and you won't get the same wonderful "crunch" you get with coarse salt.
You can also try one or a combination of seeds, such as sunflower, pumpkin, poppy and sesame. Some herbs and spices also work well as toppings.
Top the pretzels with the salt and/or seeds right after they come out of the wash so the toppings stick, before the wash has a chance to dry.