There are a set of stairs here but they are blocked by a Hiker right now. There is a Poke Ball hidden in one of the rocks to the north. Go west across the next bridge to find a Poke Ball that is only accessible during the Autumn when there are fallen leaves to walk on. Northwest of the Poke Ball is a path that leads to a hidden Potion also only accessible during Autumn. Finally, head north to Floccesy Ranch.
Items in Route 20:
North of Lass Isabel.
Poke Ball (Hidden)
Hidden in a rock north of the Hiker blocking the path.
West of the Hiker blocking the path. (Autumn Only)
On the path west of the Flocessy Ranch entrance. (Autumn Only)