Hayfever is a wonderful indie game on Steam. here is a guide to help you find all locations of hidden secrets in the game.
Summer Day 11 – Red Rail (Awoo!)

Go down through this narrow opening in the thorns.
Follow the titular red rail to the right.
Summer Day 15 – Rainbows (Kindness)

Use the rainbow power-up to go straight up along the wall.
Summer Day 20 – Flower Power (Knasibas)

Wait a bit until this flower has spit out a long trail of clouds, then use it to go up.
Summer Day 27 – Orange Leaves (Koni’s Hideout)

Use the rainbow power-up to go up along the wall to find an opening. Follow the path through the opening.
Fall Day 5 – Chewtoy (Jonas)

Get on top of the platform above by using the balloon power-up and the red balls thrown by the scarecrows.
Use the balloon power-up to fly further up.
Winter Day 5 – Icy Winds (Mighty Goose)

Go past Carlie and jump down to the right.
Winter Day 6 – Throw A Kiss (Bhomas)

Use the balloon power-up to fly straight up past Carlie, to an opening on the left wall.
Winter Day 8 – Floaties (Samohb)

Land to either side of Carlie, and follow the upward arrows.
Winter Day 9 – Motion Sickness (Beldrama)

Fall through the opening on the lower right corner and continue to the right.
Winter Day 14 – Tower Of Pollen (Filip)

Enter the suspicious looking block on the right wall to get a sneeze charge. With three sneeze charges, jump up to the low gravity area above Carlie.
Winter Day 23 – Spinys (Kit)

Use the rainbow power-up on the lower right side to fly straight up.
Spring Day 14 – Sticky Switches (Pablo)

Use the sticky power-up to stick to the ceiling and follow the wall upwards.
Spring Day 26 – Bounce Button (Nora)

Go through the small opening in the wall on the left side.
This guide is currently incomplete and work-in-progress. we will update later.
By mooware