My favorite pack for around the city is:
REI Flash 18 Pack $34 - super light rucksack, fun colors (but also stealthy black), I carry a 32 oz water bottle inside mine
http://www.rei.com/product/827110/rei-flash-18-packThey recently had the older model on sale for $20 but these new ones are still a deal at $34. I abuse mine and they hold up great.
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Dagny, you just sparked an idea! I've got a small camelback knock-off that might work well. (It's the same colour as the REI pack in the link.) I'm found of backpacks, even though I'm always tempted to overpack!
My other city pack is a Camelbak H.A.W.G. -- good choice. In summer, if I'm going to be ducking into a cafe or other air conditioned places, it is nice to have a light jacket so have to have some space (I carry a super light Marmot water resistant windbreaker with hood).
You may not be as vulnerable to afternoon thunderstorms as we are - here the weather can turn very quickly on a sultry summer day.
I also carry a little AM-FM radio in the city.
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