Brynna Plain is a small grassland in Breath of the Wild. It is immediately north of Lanayru Bay, west of Horon Lagoon, and south of Ja'Abu Ridge.
Breath of the Wild
An enemy base can be found in the center of the plain, containing three Green Lizalfos and three Blue Lizalfos. Once defeated, a treasure chest will activate, containing a Silver Rupee. A second treasure chest can be found just east of the enemy base, at a lower elevation. Blowup the boulders against the wall to find the treasure chest which contains a Fisherman's Shield. On the northwest side of the plain, there are some sparkling leaves moving around. If Link catches them, he will find a Korok.
The Bryanna Plain is a very windy place to be, with wind coming from the Horon Lagoon to the east. The Master of the Wind quest side quest can be found here, revealing the Shai Yota Shrine. Additionally, at the northwest side of the plain, there a massive wind geyser that Link can use his Paraglider on, allowing him to float up to a higher elevation along the Ja'Abu Ridge.
Nearby Korok Seeds
Catch the fairy lights.