Preparations for outdoor grilling
A decent grill.  Yes, really.  If your park has one of those crappy little close-to-the-ground ones (instead of the great giant cast-iron ones in the national parks) you're in for an afternoon of heartbreak and frustration as you try to grill on it.
Quality charcoal.  Hardwood is more expensive, harder to light, slower to heat up but burns hotter and tastes good; briquettes are easy-lighting but don't add much to the food.  Best results: half and half.
Handi-wipes to get the charcoal off your hands.
A "chimney"-style charcoal starter, and some newspaper to use in it.  "Easy-lighting" charcoal isn't.
A "fireplace" lighter.
Long-handled metal tongs and a long-handled, wide, heavy metal spatula.
Oven mitts or thick leather gloves in case the handles aren't long enough.
Spray bottle full of tap water for putting out flare-ups, clothing, and hair.
Meat, fish, chicken, and/or vegetables, preferably some which have been marinated in a sauce or rub for a while (several hours for meat) beforehand.
Serving plates and eating plates and other picnicware.  No, you can't put the cooked meat back on the dish the raw meat was marinating on.
Chips &amp; dip or salsa for while everyone is waiting hours for you to get the charcoal going.
Bread and side-dishes to go with the barbecue, or more likely to be eaten instead while everyone waits for the meat to be done.
Headlamp flashlight for you to use while it's getting dark because the charcoal isn't ready yet and you can't see if the food is done or not.
Beer.