Not intended as a spoiler, but if you want to maximize the rewards from cairns and puzzles, here's a quick overview:
The reward will always be useable (albeit not necessarily useful) for your calling (your role is a different matter). Mostly armor pieces, but I've gotten weapon drops as well.
The item can be anything green to purple, but blue's are most common and purples drop more than green (at least in my limited experience).
The cairns/puzzles in a given zone have a level "cap." If you do them more than 1-2 levels below the cap, you'll get an item at your present lower level; if you wait till past the cap, the level does not go up (hmm...that's why it's a cap, I guess 8p).
Level 30 gear (lowest cap): Silverwood, Freemarch, Lake of Solace (yes, actually in the lake, hooray for waterbreathing necks) - 13 cairns, puzzles, treasure chests total (if I can count, which isn't a given).
Level 35 gear: Gloamwood, Stonefield, Scarlet Gorge - 7 items (2 cairns in Stonefield).
Level 40 gear: Scarwood - 2 items.
Level 45 gear: Moonshade (OMG bring a buddy to the puzzle), Droughtlands - 4 items.
Level 50 gear: IPP, Shimmersands, Stillmoor (bring a buddy to this puzzle too imo) - 6 items.
As you can see, couple these with Rare Planar goods gear and dungeon drops as you level and you'll be mostly Blue/Purple all the way from 30 to 50.