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: POKéMON Center
Location: Petalburg City
Purpose: Speak to the man near the PC to change your trainer profile.
Place: House next to the gym
Location: Rustboro City
Purpose: Trade a Slakoth for a Makuhita
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: 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
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: House next to the POKéMON Center
Location: Fortree City
Purpose: Trade a Pikachu for a Skitty
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: E-Battle House
Location: Mossdeep City
Purpose: The place to fight the trainers you swiped in with Battle-e Cards
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 Bellossom for a Corsola
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