One of the easiest side dishes to throw on the grill when we're barbequing dinner is a pan of black beans. It's a favorite for the whole family, and you can pretty much keep the ingredients on hand at all times.
1 can (or 2) of black beans (I buy them by the case at Costco)
onions (we freeze chopped onions when we only use half fresh for a recipe)
ham or bacon (just a slice or two to provide some fat and additional flavor)
jalapeno pepper (optional, and can also be stored chopped frozen)
Put all of the items together in a foil loaf pan and plop on the grill while you cook your other items. They will come together with a ton of flavor and easy cleanup!