Welcome to the guide for finding all (hopefully at some point), hidden items. If you are aware of any hidden items or anything that is hidden really, that I have not shown in the following list, do post a screenshot of where that item may be, regardless how bad it may seem.
The routes which you can find these hidden items, aswell as other things like abandoned pokemon will be shown in order based on the storyline from top to bot, first to last.
Viridian Forest - Rare Candy
Route 3 - Stardust
SS Anne 1F Kitchen - Great Ball, Hyper Potion
SS Anne 2F - Leftovers
It's in the trash.
SS Anne Ballroom - Leppa Berry, Chesto Berry, Oran Berry
Route 11 - Escape Rope
Route 8 - Lost Growlithe
Celadon City - PP UP
Rocket Hideout B1F - 2 Super Potions, Hyper Potion
Rocket Hideout B2F - Iron
Rocket Hideout B4F - 2 Revives
Route 12 - Rare Candy
Route 13 - Calcium, PP UP
Route 14 - Zinc
Route 17 - Rare Candy
Route 17 - PP UP
Route 27 - Rare Candy
Sea Foam B4F - Nugget
1. Rare Candy
2. Hyper Potion
Cerulean Cave 2F:
1. PP Up
3. Ultra Ball
4. Full Heal
Goldenrod Underground Path - Full Heal, 2 Max Ethers, 2 Antidotes
Underground Warehouse - 5 Great Balls
Goldenrod Underground Basement - Bright Powder
Goldenrod Underground Basement - Miracle Seed
National Park - Full Heal, Super Potion, TM28 Dig, Soothe Bell
Route 36 - Ether
Ecruteak City - Ether, Hyper Potion, Ultra Ball
Burned Tower Top Floor - Ether
Burned Tower Floor 2 - Nugget, Revive
Route 39 - PP Up
Olivine City - Pearl, Protein, Rare Candy, and a lost Staryu
Violet City - Rare Candy
Golden Rod - Abandoned Poké - Spinirak
(You surf from the rocks at north of the city, and then all the way down.)
Route 35 - Nugget
Mt.Silver 2F - Rare Candy
It's somewhere under the player inside the rocks.
Five Isle Meadow - Pearl
Water Path - Pearl
Five Island Rocket Hideout - Leftovers
It's inside the trash.
Violet City and Route 44 - Hyper Potion, and PP Up
Amazon Forest - Oil Drum has a Poison Barb
The guy is hiding in a box. That's it.
Some man hiding from his wife in Safari Zone, Kanto. He's selling Shiny Stones for 20000$. Specifically Safari Zone Area 1
"I am talking to a tree... I need to see Nurse Joy..."