2L Camelbak with:
"Two external pockets for small items like keys, wallet, phone or storage for a shed layer.
Stretch elastic side panels provide for expansion; twin compression straps shrink pack down when empty. Reflective back strip with tab to click on a blinker for early morning and evening safety."
Some places sell this for ~£45, others up to £59 (Ellis Brigham)
I bought one of these last year for Burning Man and it stood the test very well. The pockets were very handy and I could put an additional bottle in the elasticated pocket.
One pocket has a hook for keys etc.
Picture on Decathalon seems to be the older version of the Rogue compared to Camelbak US site - seems it's recently been altered. Picture is the version I bought.