For NGOs & festivals: You must make sure that all the necessary equipment to implement the project is available. Festivals and NGOs should agree on which resources the festival will provide and which the NGO needs to bring.
For NGOs: Besides the equipment, you also need personal items. This includes everything from IDs to clothes, toiletries, medication and electronics. It is never a bad idea to bring clothes for all kinds of weather, especially rain (and any subsequent muddy ground).
Please also make sure you don't bring any prohibited items. Festivals usually list them on their homepage before the event. This usually includes weapons, glass bottles and pets. Always take a look at the festival website to make sure you know about special dos and don'ts.
For festivals: Communicate early, which items NGOs aren't allowed to bring to the festival site. You should also tell them what will be available and what will not be, i.e. sound equipment, electricity, heating, lighting, stationary, etc.
Check this packing list from festival magazine Höme to prepare (German):
https://hoemepage.com/festival-packliste/FinnTheInfinncible (Finlay Games) explains “What To Pack For A Music Festival” in this video (English):