So, a big jam is set in motion for tomorrow but you have no idea what to bring? Here's a short list of items that should be brought to all jams big or small. It is essential to have almost every item as they will help make your life easier as well as others.

Water
Water is a first. When you're out drilling with your friends, your throat gets a little dry. You may even start getting dehydrated in the hot sun. A bottled water would be great to take with you because it'll help make your jams last much longer if you didn't have to search for a fountain.
Clothes
Second, loose but comfortable clothes is a must for any athlete to train. Plus, it'll protect your body from being injured through rolls and those random cuts you find on your arms and legs. It is also necessary to bring clothes for cold weather throughout the winter season.
Cell Phone
It can be difficult to find a group of traceurs over such a large area. That is why I recommend you to bring a celluar device. In addition, you can switch numbers with new traceurs or freerunners you meet or you may even end up calling the hospital if someone becomes injured.
Snacks
Even though you may have had breakfast, you'll definitely get hungry after a few hours of parkour or freerunning. Small snacks such as a bag of chips, a cereal bar, or anything else to munch on would be great.
Wallet
Having your wallet serves many purposes. First, you should carry about ten dollars (Depending where you live) to purchase food, drinks, or means of transportation. Second, an ID card with your name, address, home phone number, allergies, and a few emergency contacts . It rarely happens, but someone will get hurt. In case of any emergency, those who are jamming with you will be able to find everything they need in one spot.
Backpack
How else would you carry everything else? Water bottles, extra clothing, your cell phone, a few snacks, and your wallet will fit perfectly in a small school size backpack. A group of your friends can all put their belongings in the bag as well as every single person takes turns carry the bag.