I have used this Lowepro Daypack:
Lowepro Daypack DSLR Bag (LP36267) : DSLR Backpacks - Future Shop which I believe is the stripped down version of this:
Lowepro - Orion DayPack 200 The former lacks the mesh pouches on the side of the pack and no slip locks on the shoulder straps.
In the lower camera compartment, I had a K20D with FA 28-70/4.0, Samyang 14/2.8, Tamron Adaptall-2 90/2.8 macro, Tamron Adaptall-2 180/2.5, Pentax 1.7x AFA, spare caps, lens cloth and a small rocket blower bulb. In the upper compartment, I had Sigma AF 400/5.6, Tamron Adaptall-2 300/2.8, a stainless steel bottle of water along with about 6 granola bars. The 300/2.8 was stuffed into this lens case:
SAFROTTO Protector Padded Camera Lens Bag Case Pouch cover E18 270 x 130x 130 mm | eBay It's a very snug fit to squeeze the lens into the bag sans the lens hood (I strapped the lens hood to the velcro slip lock tab). For the days when I am not packing around the 300/2.8, I've added this:
http://www.ebay.ca/itm/250880203745?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 to the upper compartment for securely carrying smaller lenses and accessories.
This budget daypack is fairly well constructed but with minimum amount of padding and all the weight can be felt. I wouldn't go on a long and strenuous hike with this daypack but in a pinch it can do the job. I am hoping to find the Orion Daypack 200 in the local stores to check out the build quality before buying it but for now the budget daypack from Future Shop is adequate for most situations.
I use the bottom compression straps to either carry a small tripod or a monopod almost always. I'll add some photos of my set up if anyone requests them.