Get a free 500ml bottle of water at Boots with the O2 Priority app. If you don't have O2, you can get a Free sim card on their website.
The Freebie comes in three flavours: Cucumber & Mint, Ginger & Lemon and Melon & Jasmine.
Be careful when you click the 'Use Now' button, you'll only have 5 minutes to get to the till and get it. So only press it when you're at checkout. Good Freebie.