Originally Posted by
Bacciagalupe
If you're cycling/touring offroad, a backpack can work OK.
If you're on the roads, wearing a big backpack the whole time will suck. Period.
Since you will be hiking for literally one day, you're better off with an ultralight frameless daypack like the Marmot Kompressor. Roll it up and keep it in your panniers during the trip, and break it out when you're doing that one-day hike or if you need a pack while wandering around a town. Weighs only 283g, volume 15 liters. The Plus is 340g and 18 liters.
First off, I have no panniers. It's a lot of weight, throwing a pair of Ortliebs on there, when I probably can't fill one to capacity with my stuff. But I get your point.
I wouldn't dream of hiking in the Kompressor. It's easy to say "one day" but if you're miserable with 14 pounds on your back for 5 hours, suddenly I'd wish I had brought something more than a dry-bag.
The Repack-15 has the same capacity, and it adds less than 300 grams for a top-rated foam sheet suspension that air-cools. I'd say it's a step up your suggestion. I think I will, however, try to heed the warning and strap it to my rack rather than my back.
Except...
I saw a warning on a photography forum that bike racks are subject to high-frequency vibration that can ruin cameras, in which case I'd absolutely wear a backpack (it's not THAT bad, it's just a backpack, you sweat anyways...). Has anyone heard anything to corroborate camera safety as a reason to use a bag?