The recent trailer pretty much showed everything for Isle of Armor - it's basically the one quest of "Go here and watch a cutscene" and takes an hour to beat. Then it's just a slightly bigger wild area.

Crown Tundra is larger, both in story and 'wild area' scale, as well as replayability. When you arrive in the Crown Tundra, you'll be welcomed by a random legendary bird who you fight. The other two birds interrupt, then Calyrex stops them fighting and makes you fall asleep. You wake up in Peony's house, him and his daughter having found you on an expedition. When you explain what happens, he urges you to join is exploration team as he's been researching the legendary birds in the Crown Tundra. After the main story, Peony will give you daily missions to find items, catch specific Pokemon, and eventually find more legendary Pokemon to earn more points in the exploration team (which can be exchanged for TMs, items, etc).

Calyrex has another form, where the bud on its head opens and drapes around him like a cape. His legs become longer, and his rabbit head now has antlers that form the shape of a crown.

Part 3 (The Cinder Citadel) will release next February as part of Pokemon Day, and is set in an island off the west of Galar filled with rolling hills and castles, where a strange group enlists you to piece together the mystery of a fabled dragon that mysteriously vanished hundreds of years prior.