At Zion I remember I really enjoyed the Weeping Rock trail - it was a short hike but I thought very pretty and unique. Afterwards it was closed due to a rockslide, though, so I'm not sure when it will open up again or if it already has. And at Bryce one of my favorite trails was the Queen's Garden Trail (as you could probably tell from my story). But of course both parks have tons of amazing trails, and you probably can't go wrong with any of them. Same for the Grand Canyon!