Plenty of people have offered maps and guides to those seeking chests -- I found this map on Reddit particularly helpful myself. The moss-covered chests are limited to once per player, and will not respawn -- the same goes for the Blazing Chest found in Ordon Sanctuary, and the Smouldering Chests found on the way there. Sturdy Chests also do not respawn. In fact, there are only six chests total on the Timeless Isle that respawn not on a daily, but a weekly basis -- and there's a trick to getting them all.
- The Gleaming Treasure Chest can be found just north of the Firewalker Ruins, but it requires jumping on pillars to get to it. Just head up to the Ruins and leap across the pillars to the north of the Ruins to obtain it.
- The Rope-Bound Treasure Chest can be found by doing what else -- climbing across ropes that are suspended high above three caves in the valley below. The entry point for rope-climbing is just to the right of the first bridge past The Blazing Way -- carefully make your way across the ropes to find the chest on a pillar below.
- The Mist-Covered Treasure Chest can be found high in the air above the plateau just before the final bridge to the Ordon Sanctuary. Access these chests -- there are more than just one -- by clicking the Crane statue on the plateau.
Three other chests are available for weekly looting as well -- the chest in the sunken pirate cave on the west side of the island, between the Horde and Alliance flight paths, the locked chest on the sunken ship to the south of the island that requires killing mass amounts of Hozen Swabbies to obtain a key, and the satchel that is dangling from the mast of the pirate ship to the southeast of the island. Those chests can be obtained in any particular order.
At the moment, it seems as though all the chests will give you upon repeated looting is simply more Lesser Charms of Good Fortune, along with Timeless Coins -- but it is a fair number of coins. Players collecting coins for the various gear upgrades, vanity items, and other things offered by vendors on the Timeless Isle should probably make these chests a weekly priority. It's one of the easiest ways to get Timeless Coins, and you don't have to kill any pesky elites to get them.