So, if you're into fashion you may hate this input, but we live fairly close to NYC and when we visit there, I generally find it most comfortable to carry a backpack. I just find it more comfortable to walking and standing to have the weight evenly distributed and it frees my hands up for hand-holding with the kids.
The one I have and love is actually a critter back-pack from LL Bean (in a discontinued green color that I love).
http://www.llbean.com/llb/shop/64782...al%20Red/Snake
But I'm sure you can find a more adult-looking pack with similar dimensions.