I have a 2 litter Camel Back in a small hydration pack, which is okay. I only use this pack in hot weather on short hikes, due to the small capacity of the pack itself.

When I recently upgraded my regular Day Pack to a Bergan's of Norway Pack which was hydration compatible.

After doing some research and talking to some people who used them I went with the 4 litter MSR Dromlite Bag for my day pack. You can get the hydration hose or the shower attachment for both the Dromlite and Dromedary bags. The threads are compatible with Nalgene wide mouth lids. I actually went with the shower attachment as I hike with my dogs and wanted a way to cool them off if needed.

http://cascadedesigns.com/msr/water-trea...te-bags/product

They also make this heavy duty version

http://cascadedesigns.com/msr/water-trea...ry-bags/product

and the hydration bladder version of the Dromlite.

http://cascadedesigns.com/msr/water-trea...-system/product