Here's a couple of small packs I recommended. When I just want to carry the bare minimum, my lightweight bag is a Lowepro Fastpack 100. I chose this one, because I hate the single sling setups. With the sling, there's just too much weight on one shoulder. Using the double straps evens out the weight. The really simple design also doesn't scream 'Look I have a camera in here'. I'm able to fit a Canon DSLR with a 24-70mm in the bottom area and another lens in the top part. Plenty of space for media cards, lens cleaners, and filters. If you're shooting with smaller cameras, there's more than enough room in the 100, but of course there's larger versions available too.
The Fastpack 100 is a great small pack (I actually have two), but the Flipside versions from LowePro are just a bit larger and the next step up to being able to hold a Tripod and a long lens. It's called the Flipside because of the way you gain access to the gear which is both functional for keeping things out of the dirt, and safer from pick pocketing sticky fingered thieves. If you need to hold a Tripod, carry longer lenses, or external Flashes, the Flipside 200 would be a good start. There's also larger versions of the Flipside available too.