Also, pack plenty of dry snacks and a few dinners that don't need to be cooked (peanut butter and jelly sandwiches, for example), in case you find yourself having to feed hungry mouths during a downpour. Candy bars or trail mix are great for energy replenishment between meals, but store them in a cool, shaded spot so they don't melt and ruin other food.
Kill the Music
It's tempting to turn car camping into a tailgating event—grilling food over a flame with your buddies and blaring The Grateful Dead's Europe '72. There's nothing wrong with that, but keep in mind that sound travels incredibly well through patches of open space like campsites and lakes. While you might be enjoying the slow jams, you could be infringing on someone else's peace and quiet and ruining the chance of seeing any wildlife.
Bring Multiple Fire Starters
I once read that when traveling abroad, it's wise to keep some cash in at least three separate spots. The same could be said for fire starters when car camping. Separate your fire starters to ensure that you'll always have another spare, dry option if one set gets lost or soaked.
I like to bring a few different types—matches in a Ziploc bag in my backpack, a butane or weatherproof lighter with the pots and pans, and more matches in my car's glove compartment. If you're camping during the rainy season, it's sensible to bring a little dry kindling in your trunk in case you're stuck with damp wood.
Plan Wisely for the Rainy Days
If you're going out for more than a few days, particularly in the spring or summer, the chances are good that you'll encounter inclement weather at some point. For many people, that means hunkering down in the tent or car and waiting out the storm. If you're camping with adults, it's easy to just doze off as a group.
However, if you're camping with children, they'll want to be entertained during the bad weather, so you should be prepared. A deck of cards can do wonders for your group's boredom, as can a book or a small musical instrument like a harmonica.