Trilby Plain is a location in Breath of the Wild.
At the far east of the valley, there are some ruins scattered about. At the northeast building, there is a treasure chest here which contains a Ruby gem. Also in these ruins there is a small metallic block. Link can use Magnesis to grab the block and put it in its right location on the wall to the northeast to get a Korok Seed. At the very far east, up on the higher ledge, there is an enemy camp with several variants of Bokoblin. At the top of the camp, there is a wooden treasure chest with a Topaz gem.
Near the path, just east of the Thims Bridge, Link will find a small camp where Fin is nearby. Him and his friend Russ run a Shield Surfing business, where they collectively refer to themselves as The Shield Breakers. If Link talks to Fin he can quickly appear at the top of the hill where Russ is located. Russ sells shields that Link can use to shield surf.
In the tree right near Russ, there is a small stump that Link can step on, causing a yellow circle to appear near the pond to the northwest. If Link shield surfs fast enough and reaches the circle, he can get a Korok Seed. Also in the pond, there are some sparkling leaves. Link can catch them to find another Korok.
Nearby Korok Seeds
Uphill to the southeast is a shield vendor who talks about shield surfing. This should be a hint. In the tree above him is the seed platform to start the race. Shield surf down to the marker for your seed.
Solve the block puzzle.
Catch the fairy lights.