There's plenty of places that are useful but are easily forgotten. Below is a list of these out of the way spots for the Hoenn region.
Place: Altering Cave
Location: Route 103
Purpose: After you become the Champion this cave is opened up. As it was in Fr/Lg, only Zubat roam here until....?
Place: POKéMON Center
Location: Petalburg City
Purpose: Speak to the man near the PC to change your trainer profile.
Place: House above the POKéMON Center
Location: Rustboro City
Purpose: Try to make the baby girl laugh
Place: House next to the gym
Location: Rustboro City
Purpose: Trade a Ralts for a Seedot
Place: Devon Corporation
Location: Rustboro City
Purpose: Have your Claw or Root fossils revived
Place: Dewford Hall
Location: Dewford Town
Purpose: The place to see what the latest trendy phrases are
Place: Medicine Shop
Location: Slateport Market
Purpose: Trade powder you made with the Berry Crusher for items.
• Energypowder, Heal Powder (50)
• Energy Root (80)
• Revival Herb (300)
• Calcium, Carbos, HP Up, Iron, PP Up, Protein, Zinc (1,000)
Place: POKéMON Fan Club
Location: Slateport City
Purpose: Receive the Soothe Bell and Scarfs
Place: Lady next to the Energy Guru
Location: Slateport City
Purpose: She'll let you know wether or not you've maxed a POKéMON's EV's and if you have, give the PKMN an Effort Ribbon
Place: Cycling Road
Location: Route 110
Purpose: Take the Mach Bike challenge!
Place: Daycare & Breeding Center
Location: Route 117
Purpose: Leave POKéMON to level up or possibly breed
Place: Right-most house
Location: Verdanturf Town
Purpose: The lady inside will tell you how much the first POKéMON in your party likes you
People: Gabby & Ty
Location: Route 111, 118, 120 (eventually only 111)
Purpose: Defeat them to get on TVs across Hoenn
Place: Glass Workshop
Location: Route 113
Purpose: The place to buy flutes & two rare base decorations but only if you've walked in enough soot
• Blue Flute (250 steps)
• Yellow Flute, Red Flute (500 steps)
• White Flute, Black Flute (1000 steps)
• Pretty Chair (6,000 steps)
• Pretty Desk (8,000 steps)
Place: Move Tutor's House
Location: Fallabor Town
Purpose: Trade Heart Scales to allow a POKéMON to remember a previously forgotten level up move
Place: Fossil Maniac's House
Location: Route 114
Purpose: After becoming the Champion you can reach the Desert Underpass through the Maniac's house to find wild Ditto & the Claw or Root Fossil
Person: Elderly Lady near the Cable Car
Location: Mt. Chimney
Purpose: Buy Lava Cookies (200)
Place: Herb Shop
Location: Lavaridge Town
Purpose: Buy bitter health items
• Energy Powder (500)
• Energy Root (800)
• Heal Powder (450)
• Revivial Herb (2,800)
Place: Weather Institute
Location: Route 119
Purpose: After becoming the Champion speak to the institutes director to find out where Groudon & then Kyogre might be
Place: House next to the POKéMON Center
Location: Fortree City
Purpose: Trade a Volbeat for a Plusle
Place: POKéMON Center
Location: Lilycove City
Purpose: Three Ladies
Contest Lady: Give her the correct flavor POKéBlock for her POKéMON. Give the right one and check the tvs later. She'll give you a shout-out after she wins a Contest.
Quiz Lady: She'll ask you 1 of 16 questions. Answer correctly and win a prize.
• Which isn't a game version? - Dark / Bead Mail
• Which is the pretty item? - Flowers / Max Ether
• Which stores your POKéMON? - PC / TM 15 (Hyper Beam)
• Which will stop poison? - Immunity / Durin Berry
• Which item do you break? - Egg / Max Ether
• Which stores information on POKéMON? - POKéDEX / Glitter Mail
• A POKéMON was once this. - Plush Doll / Star Piece
• How do POKéMON evolve? - Level / Tropic Mail
• Steel is strong versus which? - Ice / Rare Candy
• Ghost is weak versus which? - Psychic / Premier Ball
• Dark is weak versus which? - Fighting / Rare Candy
• Which of these is weak? - Rock / Watmel Berry
• Which will block escape? - Shadow Tag / Belue Berry
• Which goes with center? - POKéMON / Luxury Ball
• Which will stop confusion? - Own Tempo / Max Ether
• Which machine gives you information? - Television / Big Pearl
Favor Lady: She wants something but isn't quite sure what. Give her the right item and she'll reward you with one of four vitamins.
• Something slippery - Repel, Super Repel, Max Repel, Antidote, Paralyze Heal, Burn Heal, Pecha Berry, Grepa Berry, Belue Berry, Awakening, Fluffy Tail / Protein
• Something roundish - Energypowder, Rare Candy, Pearl, Big Pearl, Hard Stone, Smoke Ball, Revive, Max Revive, Shoal Shell, Tinymushroom, Aspear Berry, Oran Berry, Grepa Berry, Magost Berry, Watmel Berry, POKé Ball / Calcium
• Something wham-ish - Revival Herb, Potion, Fresh Water, Soda Pop, Lemonade, Hard Stone, Light Ball, Lava Cookie, Chesto Berry, Nanab Berry, Wepear Berry, Kelpsy Berry, Nomel Berry, Durin Berry / Iron
• Something shiny - Heal Powder, X Speed, X Attack, X Defend, Blue Flute, Yellow Flute, Red Flute, White Flute, Black Flute, Nugget, Sun Stone, Stardust, Star Piece, Pearl, Big Pearl, Twistedspoon, Silverpowder, Brightpowder, Luxury Ball, Premier Ball / Zinc
Place: Dept. Store
Location: Lilycove City
Purpose: POKéMON Lottery
• Master Ball - Match all numbers
• Max Revive - Match 4 numbers
• Exp. Share - Match 3 numbers
• PP Up - Match 2 numbers
Place: POKéMON Trainer Fan Club
Location: Lilycove City
Purpose: Find out just how many fans you have
People: Cove Lily Motel
Location: Lilycove City
Purpose: Chat with members of the Game Freak staff to get your diploma if you've completed your POKéDEX
Person: Diving Treasure Hunter
Location: East edge of Route 124
Purpose: Trade shards for evolution stones
• Blue Shard - Water Stone
• Green Shard - Leaf Stone
• Red Shard - Fire Stone
• Yellow Shard - Thunderstone
Person: POKé Block tutor
Location: Mossdeep City
Purpose: Tells you which color of POKé Block the first PKMN in your party prefers
Place: Wireless Mini-games House
Location: Mossdeep City
Purpose: Try out POKéMON Jump & Dodrio Berry-Picking with friends
Place: Space Center
Location: Mossdeep City
Purpose: Find out how many weeks it's been since you started your current game
Place: House northeast of the POKéMON Center
Location: Sootopolis City
Purpose: Show the brothers a Shroomish or Barboach over 15.7 inches to win Elixers
Place: Middle house south of the POKéMON Center
Location: Pacifidlog Town
Purpose: Trade a Bagon for a Horsea
Place: House southwest of the POKéMON Center
Location: Pacifidlog Town
Purpose: Show the man your lead POKéMON and he'll either give you TM21 (Frustration) or TM27 (Return) once a week
Place: House to the left of the Battle Tower
Location: Battle Frontier
Purpose: Trade a Skitty for a Meowth