Sun tan lotion! You'll be out all day, and you will know all about it when you get home.
Aside from what has already been mentioned, here are a few(!) things I carry in my car (ours is a volunteer & weekends only club). I soon learned not to rely on others to bring these

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- Comprehensive toolkit (spanners, allen keys, screwdrivers etc.)
- Grease, 3in1 oil, WD40
- Glider retrieving rope (goes on towing eye or towbar of car & has two chain links at glider end to fit tow hook. Rope length at least 1 1/2 times length of one wing, 3/8" polypropylene rope)
- White PVC insulation tape (for sealing gaps when putting gliders together)
- Footpump &/or compressor
- Adaptor to fit glider tyre valves
- Jump leads
- Mobile phone & list of other members phone nos.
- Wellies (If you get the kit out early when the grass it still damp, you don't want your feet wet for the rest of the day)
- Overalls & disposable gloves (even if you have just fixed the winch or retreive vehicle, your black fingerprints won't be appreciated on white gliders)
- Watch
- Digital camera
- Paper towels or blue kimwipes. Tissues don't stand up to a full day's runny nose...

If you want to be popular, bring a big pack of broken biscuits and a pint of milk for the coffees at the launchpoint

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Some glider pilots can be a bit cantankerous, so above all, bring patience and a sense of humour!