The Crown Tundra is the second half of SW/SH's DLC, and offers a more legendary focused experience than Isle of Armor. Peony and Calyrex are great new characters you meet in the main legendary storyline, making up for the fact this DLC is much less NPC focused. While you'll be talking to Peony and even Sonia as you hunt down legendaries, they'll only really have a few comments to make. Instead of that, the DLC is much more focused on Pokemon itself - in the snowy wild area, there's 100+ new (to the game) Pokemon to discover, with the main attraction being the legendaries. The Galarian forms of the legendary birds must be hunted down after you startle them during a scuffle at the mysterious giant tree, the Regis are locked behind temples with each one having a puzzle you must do before they open for you, the Three Musketeers require you to track down their footprints so Sonia can narrow down their location (with Keldeo having a little puzzle required to catch him), Calyrex and his steed are obtained through the main storyline, and the remaining legendaries are obtained via the new Legendary Dens - pick up a rental Pokemon and traverse several raid battles in a row, swapping out for other Pokemon you catch until you finally confront the den's legendary. Thanks to that, this DLC has the potential to add a decent chunk of time to your playtime.
The new content isn't just limited to the Crown Tundra - once you've completed all the story content of CT, IoA, and the base game, you'll now get the Galarian Star Tournament. 17 different partners to choose from (most unlocked via wins in the mode) as you engage in a multi battle tournament, made for those who prefer to engage in battles rather than do raids or explore a new wild area.
Overall, Crown Tundra is an incredibly solid DLC to round out SW/SH, fleshing out an already solid game with extra content.