As the little girl said to the little boy, that's a handy thing to have on a picnic...
NO MORNING COFFEE OR TEA. Diuretics both, but coffee is the worst.
I used to sit on a large bath towel, and land in a discrete corner of the airfield. Once, when I was tending to business behind the glider, the CFI spotted me and came over to discuss my landing....quite impossible to stop in midstream, I had to listen while he discussed at length the finer points, as though there was nothing unusual about the situation. A man of dedication.
Dont overhydrate. Just carry a small bottle of water in the aircraft, drink a bit if you feel thirsty; your body will ask for water if you need it. Dress warmly if it will be cold...if I was strapped into a glider I would keep handy a small baby blanket, and cover up if cold and tuck it behind me but in reach if not needed. Wear a hat (without a visor that would obstruct your vision) if sunlight may be a problem.
My longest flight in the UK in a glider was a 511 kilometer task, took me 8 hours and 53 minutes. After landing and putting things away, a beer tasted very good indeed.
When I fly anywhere in power, I stop and refresh me and the airplane every couple of hours. That way neither of us ever ran out of fuel....