Water

Staying hydrated while on a hike is important. Drink water regularly throughout the day. Add electrolytes for longer, hotter treks. Bring more than enough and keep some in the car.

A zonked out feeling due to dehydration is not fun and also dangerous. Drink more than you think you should. You could be sweating and not even know it.

If your hike involves filtering various sources (lake, stream, pond, cow trough), a Sawyer squeeze (aff link) water filter is the way to go. Fairly compact, lightweight and used by thousands of day hikers, thru-hikers and adventurers.

It comes with bags which are good to fill up at the source to bring back to camp, but can split because of excessive squeezing. A better method is a bottle. Smartwater or equivalent. Those plastic bottles come in different sizes and the cap screws in perfectly. Also take much more use and abuse too.

Bring any sort of bottle, camelbak, bladder you want on your hike. Just always have water!