Osprey Manta 36.
It's much larger than a lot of packs, but cinches down nicely and will easily hold a enough stuff for an over-night. It's a taller pack which means it comes down lower and curves along your pack so it's not bad while riding.

Honestly, go to REI and try a few packs. The problem with most biking packs is that they're oddly shaped, and don't hold very much. A 15L pack sounds like a lot except the bladder takes ~3L, and funny pack shape means you only have a ~8L left to work with. My ~12L Camelbak is basically full with only tools (just basic kit, medium seat bag worth), water, and a light jacket.