Check out these ones:
https://www.scicon.it/transalp.php
I have a pair of Scicon panniers and like them very much (normal panniers, not these ones - I think they don't produce it anymore). Italian stuff, I suppose.

Arkel Bug.

I have used a hiking pack on top of a large custom rack on my last two long tours. I loved riding into a National Park, locking the bike, throwing the pack on my shoulders, and spending a day or two hiking in the back country.

https://www.crazyguyonabike.com/doc/springforward

Ideally a backpack should have an adjustable harness comprised of wide, padded sholder straps that distribute the weight across the back, a sternum strap to keep the shoulder straps together and a padded hip strap so that weight is carried by the hips as well as the shoulders

Unfortunately such a harness is a little bulky so most 'convertable' panniers have thin shoulder straps rather than a complete harness and are completely unsuitable for carrying anything more than a few lbs for short periods of time.

Lambert Cycles had an amazing model on the market a few years ago under the Voyager name that not only embarrased anything else on the market - it was half the price. Unfortunately it didn't sell well and was discontinued. Talked to the guys in product design at Lambert last year at ExpoCycle because customers gave been asking for it so there's a small chance it might resurface.

I was lucky enough to pick up a couple myself and they probably do more duty as a backpack than as a pannier - they're that good. But the next best thing would be a bonifide backpack dropped inside a simple pannier of similar size and then loaded from the top.

Something else similar to the ultralight daypacks that a few people have mentioned is the Outdoor Research Drycomp Summit Sack. My brother and I strapped them to the racks as a dry bag with tent/thermarest/etc in, and then used it as a daypack for little hikes or just a few days jaunts away from the bikes. Worked great as a backpack, super light, and as a dry bag actually kept things dry too. After being outside for probably about 400 days now it's started to show some wear and a few small holes have developed where the daisy-chain attaches, but overall it was fantastic for on top of a rack.

My wife got an outdoor research day back last year or so and while it doesn't have a proper waist belt, it is impressively light and compact, and has been reasonably durable as well. A good padded waist belt sure does a diff for heavier stuff and longer hikes, so as others state, its.always a compromise, but boy these newer types sure are light and compact , and or useful for on rack use. Nice to see other brands making them too.

I thought these might prove useful to the next person with this dilema down the line. Bit more heavy duty than alot of the stuff sack style bags out there. I personally use a Kelty Daypack strapped one my rack.

https://www.maxpedition.com/store/pc/...06.htm#details

https://www.maxpedition.com/store/pc/...ack-26p474.htm