I'm a big fan of the Naked Running Band, then next would be the Ultimate Direction Utility Belt. Both of these come in many different sizes so they are designed to slip on and fit securely, without buckles or adjustment straps. The benefit of these is that they do not bounce like many of the other bands. And unlike vests, waistbelts help you stay cooler and avoid a sweaty back.
I've taken the NRB out for 10+ hours at a time. I've carried a 600ml soft flask with water filter, gloves, sunglasses, windbreaker, food, satellite tracker, phone, and first aid. The only time I'll break out a vest is when I don't have access to water or need to carry ice axe, microspikes, or other things like that.
If you are just going around town, the NRB would hold 2-3 500ml soft flasks, depending on what else you carry.