Princess Peach Adventure Quest

Special Seals hidden throughout all levels and Green Stars
Collecting them all unlocks World 9

1-1, 1-2, 1-3
flower meadows

2-1, 2-2, 2-3
Rock mines

3-1, 3-2, 3-3
Tropical grape island

4-1, 4-2, 4-3
Jungle world

5-1, 5-2, 5-3
Snowfall

6-1, 6-2, 6-3
Koopa Troopa Base

7-1, 7-2, 7-3
Sugar Clouds

8-1, 8-2, 8-3
Bowser Kingdom
9-1, 9-2, 9-3
Star World (a secret world)
Oh, and lastly, each level of
1-4, 2-4, 3-4, 4-4, 5-4, 6-4, 7-4, 8-4 and 9-4
They are the final fortresses of each world
In world 8, Princess Peach is imprisoned in Bowser's castle.

Mushroom (obviously the classic one, it makes you grow and withstand one more hit)
Fire flower
Star of Invincibility
Boomerang Flower
The Super Cape (which allows you to fly, but only if you take a running start)
Raccoon leaf (Gives you tanuki ears and tail that also allow you to fly, but only for a short time)
Super Wing P (allows you to fly infinitely until the effect wears off)
Extra Life Mushroom
The Electric Gloves (Gloves that will help you open hidden paths that require an electrical frequency; this item will be obtained after completing world 2-2)
And lastly, there's Yoshi!! That loyal, cute green dinosaur who will help you too.

Goombas
Piranha plants (Both the green and red variants)
Lakkitu
Dry bones (found only in castles)
Koopa Troopa
Parra troopa (Flying variant)
Red Koopa Troopa
Blue Koopa Troopa
Bullet Bill
Spiny
Boo and Big Boo (only found in fortresses or haunted houses)
Fireballs
Sheep-Sheep (Only found in aquatic levels)
Bloopers (only acoustic levels are found)
Hammer Bros (including the green variant that throws hammers, the red variant that throws fireballs, the cyan variant that throws snowballs, the blue variant that throws boomerangs, and the fat variant that throws heavy hammers and causes earthquakes)

Bowser
Kamek
Bowser Jr
The Koopalings

Princess Peach
Toads
Mario
Luigi
Bowser
Kamek
Bowser Jr