I just got back from this area and was doing some follow-up googling to see what others thought of it.
So here is my take;
I used Tex's Riverways for logistics- an excellent no frills, no BS outfitter, I felt their prices were very reasonable.
Kelsey's guidebooks are excellent, but bear in mind, if he says 6 hours, you had better making good time to match it!
He is also rather sparse on detail- because he covers so much ground.
Jasper Canyon/Ridge was the highpoint of our trip from Mieral Bottom to Spanish Bottom- truly an epic day hike, with everything you need; a few tricky rock moves, LOTS of routefinding problems, magic scenery and varied and interesting terrain.
The entry into the Canyon from the river is about 30 ft of 5.3- a nice scramble up, but woth taking a rapel rope for coming back out- there's a good anchor rock on the ledge, with an OK sling on it.
Be aware that there are no cairns you can depend on for this route. We found many that had been kicked over all the way to Chimney Rock. I rebuilt enough to make my way back, but don't expect too much. There were none apparent on any of the routes out of the Canyon. Because of this, we hiked about two miles up canyon, and ascended the south side where it opens up along a gradual rightward(going upcanyon) bend. Lots of zigzagging, but you may see a well hidden granary for your pains.
Be very careful when if you return along the Ridge- it really is a Maze, and there are enough cairns going to Water and Shot to lead you astray. There were no indications at all for the descent into the head of Jasper, so we returned along the Ridge, and made a descent into the Canyon a little west of our ascent route. The crux is the last 30 ft, I was lucky to find a break and chimneyed down just before dark!
The systematic destruction of cairns is disturbing, I hope this is one isolated instance, maybe a misguided preservationist thinking they will 'protect' Jasper Canyon's pristine-ness. The reality is that without a marked route, hikers will spend more time, and make more disturbance as they figure out the route in this sort of country. Not to mention the safety factor.
Other hikes worth checking out;
Powell Canyon- a very pleasant route to the Confluence overlook. Well cairned, a few scrambles, and access to the trail to the Doll House and beyond.
Water and Shot- better going than Jaser Canyon, in the bottom, less deep sand, and more open. But maybe not as striking. A nice loop.
Horse Canyon- there appears to be a way up past the dry falls on the right (north). Rather sketchy use of a tree and ledges to gain the lip, I couldn't see the final 10 ft, but it was clear that someone had spent some time clearing mud and debris to make a way up.
Good luck, it's wonderful country.