1
Water Bottles
Hiding alcohol in a water bottle is a pretty common tactic that festival management will be careful to check for. So, go the full mile and replace the contents of the bottle without breaking the seal. When security sees that the seal is still on, they won't question the contents of the bottle.
2
Sleeping Bags
A pretty unique way to hide a bottle of alcohol is to tuck it in a sleeping bag and chuck it in the bottom of your suitcase. No security guy is going to go ruffling through your bag and unwrap your entire sleeping bag. There's a line behind you, too, and going through your possessions so thoroughly will just hold up the line behind you.
