Spent this week playing it, I love it. Plenty of post-game content.
I am currently going through hunting down all the legends, there are just a few I cannot find yet:
Mewtwo - in Mt. Tarot? (need to check that yet)
Suicune - in Mt. Coal? (searched everywhere incl. outside but I cannot find it)
Dialga - in Mt. Frost? (need Arceus yet)
Palkia - in Pyramid? (need Arceus yet)
Arceus - in Hall of Origins > need Azure Flute, how (Quest)?
Zygarde - I dunno (need Xerneas + Yveltal, I think).
I hope I can help, here are all the locations of all legenderies
ARTICUNO - 4-season-island (After the orbs quest, which can be completed after the E4)
ZAPDOS - 4-season-island (After the orbs quest, which can be completed after the E4)
MOLTRES - 4-season-island (After the orbs quest, which can be completed after the E4)
MEWTWO - Mt. Tarot (After the Team Rocket quest, which can be completed after the E4)
MEW - Faraway Island (Which you can travel to after the E4)
RAIKOU - Route 15 (After the E4, behind the Power Plant)
ENTEI - Mt. Tarot (After the E4)
SUICUNE - Mt. Coal (After the E4)
LUGIA - Faraway Island (See Mew)
HO-OH - Faraway Island (See Mew)
CELEBI - Nireen Forest (After the Team Magma quest, which can be completed after the E4)
REGICE - Mt. Frost (After opening their caves from the top of Mt. Tarot)
REGISTEEL - Route 22 (After opening their caves from the top of Mt. Tarot)
REGIROCK - Route 8 (After opening their caves from the top of Mt. Tarot)
LATIAS - Victory Road (After the E4)
LATIOS - Route 10 (After the E4)
GROUDON - Mt. Priest (After the Team Magma quest, which can be completed after the E4)
KYOGRE - Route 24 (After the Team Magma quest, which can be completed after the E4)
RAYQUAZA - Mt. Sax (After the Team Magma quest, which can be completed after the E4)
JIRACHI - Tower of Stargazers (After the Team Aqua quest, which can be completed after the E4)
DEOXYS - Mt. Coal (After the E4)
AZELF - Faraway Island (See Mew)
MESPRIT - Faraway Island (See Mew)
UXIE - Faraway Island (See Mew)
PALKIA - Pyramid (After catching Arceus)
DIALGA - Mt. Frost (After catching Arceus)
GIRATINA - Distortion World (Which can be accessed from Mt. Frost after the E4)
CRESSELIA - Island of the Tower of Stargazers (Which can be accessed during the Team Aqua quest)
DARKRAI - Tower of Transyl (Which can be accessed after the E4)
MANAPHY - Route 7 (After the E4)
PHIONE - Breed Manaphy
HEATRAN - Aura Cave
REGIGIGAS - Giant's shrine (Can be caught after catching Regice, Regirock & Registeel)
SHAYMIN - Route 12 (After the E4)
ARCEUS - Hall of Origin (Which can be accessed after the E4 and some simple quests)
VICTINI - The Great Pyramid (During the Team Aqua quest)
COBALION - Mt. Coal
TERRAKION - Victory Road (After catching Cobalion)
VIRIZION - Lost Forest (After catching Cobalion)
TORNADUS - Roaming (After the E4)
THUNDURUS - Roaming (After the E4)
LANDORUS - Roaming (After the E4)
ZEKROM - Raurog Island
RESHIRAM - Raurog Island
KYUREM - Mt. Frost (After the E4)
MELOETTA - Route 21 (After the E4)
GENESECT - Aqua Hideout (After the E4)
you need to get the stargazer map to get the aruze flute and then you go to the temple where roxie asked you to find and catch groudon and go deep in the cave, then use the azure flute in the middle of the room on the centre thing