I believe the "leash" part is detachable. Very small storage, as others mentioned a small toy or two can be stored in there. I would recommend a SkipHop or similar backpack if that is what you are looking for, this is more so for the use of keeping them close to you as they are learning to walk and be on their own.
I really don't know, I sent it to my Granddaughter, who has an active 2 year old, and a newborn to contend with when she goes shopping, etc. They live in another state, and I thought this would help her keep "a handle" on the 2 yr old. She told me she really likes it, and her little girl wore it a lot, and liked the monkey. Barbara Munford
The leash is detachable. it has two D ring connections on the harness, one directly on the back and the other on the right side of the lower waist belt, would like to have had three total, one on rear and one on either side. As far as space, as others have said it is more of a flat pocket, good for stowing the leash that's really about it.
The leash is detachable, but the storage space is pretty minimal. It's adorable & my daughter loves it but you'd probably be better off with an actual back pack if that's what you want to use it for. Hope this helps!