The girl in the tall grass is hunting for a Chansey, a Pokémon that once could only be found in the Safari Zone. If you happen to have one to spare, she would happily trade her Aerodactyl for it. While Chansey are rare, this Aerodactyl is the only one of its kind.
Route 13 is comprised almost entirely of a large maze of fencing. On the way through the maze, be on the lookout for hidden items.
The lone building belongs to the Fishing Guru's younger brother. Convince him that you like fishing to receive the Super Rod, the best fishing rod! This makes it possible to reel in more than just Magikarp and Tentacool, so be sure to try it out everywhere.