Checklist....
1: Car.
Fuel (can't get far without it)
Tyres (legal and air in them)
Oil (check level)
Lights (all work)
Windscreen washer fluid (remember, just having water doesn't clear the bugs)
Windscreen wipers (check condition)
Spare wheel (inflated and legal)
2: Personal.
Clothes
Waterproofs
Money
Credit/debit card
Drinks
Food
Chewy bars (good for a quick snack)
Backpack/daysack (to put above personal stuff in)
3: Journey.
Tools
AA/RAC card
Mobile phone
Litre of oil
Bottle of water (for the radiator)
First aid kit
Directions how to get there (see Brians excellent thread)
cable ties
Disposable gloves
Bulbs
Fuses
Wire
Music (CDs/tapes/8-Track)
Duck tape
4: Campsite.
Tent
Airbed
Footpump
Pillow
Tent pegs
Kitchen roll (more versatile than bog roll, also keep in plastic bag)
Sleeping bag
Camping stove (with enough gas)
Insect repellent
Something to read (car mags)
Umbrella (golf one is best)
Chairs
Matches/lighter
Torch
Suntan lotion
Coolbag
Cup
Ear plugs
Bin bags
Wetwipes
Hat
Cordless shaver
Toothbrush
And if you have room after all of that, you might be able to fit into the car yourself ;D