I'm using a Monashee backpack, which is basically a well-designed daypack, with raised padding on both straps and waist area. As long as you don't decide to take the kitchen sink along, it's not TOO bad on cooler days. I've been considering a change to a rack bag to keep the dreaded sweaty back away in the summer months, plus the air circulation is nice for keeping cool in general. MEC sells a decent rear rack for about $15 CDN, it's a welded aluminum Trace rack with no name tag, I've looked at both, and the same rack in the Trace bag is $25 at the LBS.
As for the bag, MEC also sells a Blitz Crag climbing pack for $20 that looks like it will fit well, why bother spending more for all the extra pockets and webbing we'll never use anyway? Slice the shoulder straps off, add a couple small bungee cords to the body of the bag, and you have a custom built combo with carry handle for off the bike.

Rack - http://www.mec.ca/Products/product_d...=1121008716844

Bag - http://www.mec.ca/Products/product_d...=1121008859520