What hydration system should I use for multiple activities?
If you're looking to go minimal footprint, with no storage, why not go no bag? These definitely have downsides, but they fit under anything, with any type of clothing, or sport.

Camelbak VeloBak Hydration Jersey

or 

Camelbak Racebak Hydration Vest

Downsides include water which must be cooled, or will heat to body temp.

Check out the linked reviews.