Another vote for the MR Pop Ups. I've had the original PU28 for 5 years and no complaints. It's the perfect size daypack for early season. When temps drop into the teens and single digits, and I need extra room for more layers and gear, I'll ditch it for a Sawtooth 45.
You could buy a Pintler, bag only, and switch it out to the Pop Up frame for more room also.