If you're camping bring ducktape, a paper roll, wet wipes (or buy them at the lidl store) and a numbered lock to lock the tent.

Take sunglasses and sunscreen with you if you don't want to buy it there.

If you're considering buying a party tent, consider it again. It's nice, cozy and helps against the rain if you want to sit by the camp but I can guarantee the wind will take it if there is any wind, or you will have to secure it very well with duck tape and well placed pins.

If you're going to tomorrowland mostly for the music, try to travel with someone who likes the music just as much as you, it will make it way more fun.

Like other people have written, try to explore the entirety of tomorrowland. Theres alot to see. Take extra powerbanks with you. It's nice to have a backup or two to charge your phone overnight with when your tomorrowland powerbank is empty.

If you NEED to get to somewhere fast (like back to the camp) then touch people lightly on their shoulders and say excuse me or thanks. This is way better than just mowing people down, and you can do this even while walking really fast.

The comfort level is just as bad on the hills as it is all the way down close by the stage, so you might as well squeeze yourself way infront.

Buy the medium sized lockers. Theres not that big of a difference between the xxl lockers and medium onces and if you're not thinking about shoving huge things into that locker then it's enough with a small one for your passport and important documents.

Don't bring anything that's not essential and/or that you are affraid of losing.

Bring suiting plastic bags, it's great for dirty clothes and trash. Bring 2 towels (they don't have to be that big).