If they are the typcial restaurant size or thinner, cheddar cheese works - sometimes will put bacon on top, sometimes crushed red pepper. If the fries are of a thicker variety, doused in malt vinegar with ketchup and salt.
With the warm weather coming I'm looking forward to cooking some on the grill (using aluminum foil) - with some seasoned salt. TASTY! BTW - cooking pizza on the grill is very good as well.