Have you looked a Deuter brand packs? We have one of their day backs and a hiking backpack. Both have integrated rain covers and use the AirComfort system, which is great for ventilation when you're walking/hiking/biking in hot conditions:

http://www.amazon.com/Deuter-Futura-...r+daypack+rain

Deuter Futura 22 - eBags.com

Deuter AC Lite 22 - eBags.com

Deuter AC Lite 18 - eBags.com