Sling is a good for ease of access but often the weight distribution isn't ideal if you are carrying lots of things, especially water, which is rather important when you are out and about.
I'd suggest Lowepro's video fastpack 250 aw. It's nice and big to carry the essential toiletries, a jacket, a medium sized dslr, two lenses, a gorillapod, a 15" laptop, essential documents and still have space to spare for food, souvenirs and other things. Only downside is the water bottle compartment is way too big and the zips cannot be locked. I just hook my bottle onto the tripod anchors on the side and velcro the zips together when I'm in transit.
There's also a side access for the bottom compartment so if you store things right most of the time you won't have to fumble around to get what you need. It's also all black so an added bonus of being discreet.