There’s currently a great deal at HMV, which you can get by combining the Amex Foursquare deal and the O2 Priority Moments app.
When you check in to HMV with Foursquare, you’ll get a £5 credit on your Amex account when you spend £20, and there’s currently a deal to get a £5 voucher for every £20 you spend.
Combine the two, and you’ll get £10 off your £20 purchase (admittedly, you’ll have to spend the £5 voucher later).
You’ll need to be on O2 to get the app, and you need to update to the latest version to see the deal (it’s listed under Xmas on Priority Moments if the advent calendar isn’t working properly!)