Join the Pokémon Adventure with Pokémon GO APK for Android
UPDATE: a new website chest.pokemon.com was unveiled by the Pokémon Twitter account. It has a treasure chest which updates periodically. As I discovered, it's set to update at specific intervals, concluding at 24 hours and 999 coins (1pm GMT/6am PT), with something else happening an hour later at 2pm GMT/7am PT - possibly a new trailer!