Collect the indicated number of Hogwarts Crest to unlock the corresponding bonus:

Dungbombs in prank boxes around Hogwarts: 21 Hogwarts Crests
Exploding Cauldrons in prank boxes around Hogwarts: 48 Hogwarts Crests
Love Potion in Potions Club: 98 Hogwarts Crests
More health in duels: 8 Hogwarts Crests
Even more health in duels: 71 Hogwarts Crests
More mini-crests with each cast: 129 Hogwarts Crests
Score boost in flying events: 38 Hogwarts Crests
Two Player Dueling Pack 1: Draco and Luna: 3 Hogwarts Crests
Two Player Dueling Pack 2: Crabbe and Goyle: 29 Hogwarts Crests
Two Player Dueling Pack 2: Training Ground Dueling Arena: 14 Hogwarts Crests
Two Player Dueling Pack 4: Paved Courtyard Dueling Arena: 113 Hogwarts Crests
Two Player Dueling Pack 4: The Transfiguration Dueling Arena: 59 Hogwarts Crests
Two Player Dueling Pack 5: Ginny and Hermione: 84 Hogwarts Crests

Knocking down Crests - Locate the Crest you want to knock down. Cast Wingardium Leviosa on the object you want to use to knock it down. Note: This object can be anywhere and does not have to be in throwing range of the Crest. It is recommended you use either a Dung-bomb or an Exploding Cauldron. Once the object is in the air, hold [Run]. You can now walk around the castle and grounds while still carrying the object in the air. During this time Harry will not run fast. Make sure you keep [Run] held until you get to where the Crest is located. Throw that object at the Crest to knock it down.

Easy Mini-Crests - You can get a large amount of Mini-Crests when you have used the Luck Potion and are making your way to Herbology. At this point, every time you use Diffindo to get the Mini-Crests to appear, you will get the full amount of Mini-Crests available, due to your luck. Get as many as possible at this point to help reach the 25,000 pieces goal.

Easy fights and duels - An easy way to win almost any fight or duel is to cast Levi Corpus or Petrificus Totalus. Then, just charge your Stupefy until it chimes twice, and release it. It will result in a one hit victory until later in the game, then it will require two. Note: This does not work on Fenrir Greyback.

Use the following combo to defeat any enemy. Use Expelliarmus to knock them down. Charge up your Stupefy. While getting it charged, walk directly next to your dueling opponent. Fire your charged Stupefy for a one hit kill. This trick can give you five stars for the challenge very quickly and easily.

Trophies - Successfully complete one of the following tasks to get a trophy:

Grime Buster (Bronze): Clean up after Fred and George at The Burrow.
Used the Pensieve (Bronze): View Dumbledore's first memory of Tom Riddle in the Pensieve.
Great Party (Bronze): Create a great Christmas party for Slughorn.
Good Deed (Bronze): Help the student retrieve her stolen Gobstone.
Great Mate (Bronze): Help Ron avoid Lavender.
Portrait Novice (Bronze): Used your first portrait shortcut.
Crest Collector 1 (Bronze): Complete Crest Collector Level 1.
Crest Collector 2 (Bronze): Complete Crest Collector Level 2.
Crest Collector 3 (Bronze): Complete Crest Collector Level 3.
Crest Collector 4 (Bronze): Complete Crest Collector Level 4.
Crest Collector 6 (Bronze): Complete Crest Collector Level 6.
Crest Collector 7 (Bronze): Complete Crest Collector Level 7.
Crest Collector 8 (Bronze): Complete Crest Collector Level 8.
Crest Collector 9 (Bronze): Complete Crest Collector Level 9.
Crest Collector 11 (Bronze): Complete Crest Collector Level 11.
Crest Collector 12 (Bronze): Complete Crest Collector Level 12.
Crest Collector 13 (Bronze): Complete Crest Collector Level 13.
Potions Beginner's Luck Badge (Bronze): Earn the Potions Beginner's Luck Badge.
Speedy Brewer Badge (Bronze): Earn the Speedy Brewer Badge.
Potions Club Star (Bronze): Earn the Potions Club Star Badge.
Potions Club Veteran (Bronze): Earn the Potions Club Veteran Badge.
Duelling Beginner (Bronze): Earn the Duelling Beginner's Luck Badge.
Reflex Badge (Bronze): Earn the Reflex Badge.
Duelling Champion (Bronze): Earn the Duelling Club Champion Badge.
Duelling Veteran (Bronze): Earn the Duelling Veteran Badge.
Quidditch Beginner (Bronze): Earn the Quidditch Beginner's Luck Badge.
Precision Flyer Badge (Bronze): Earn the Precision Flyer Badge.
Combo Flyer Badge (Bronze): Earn the Combo Flyer Badge.
Keen Flyer Badge (Bronze): Earn the Keen Flyer Badge.
Quidditch Captain (Silver): Assemble the Gryffindor Quidditch Team.
Hidden the Book (Silver): Hide the Half-Blood Prince's Potions Book.
Portrait Pro (Silver): Use all of the portrait shortcuts.
Crest Collector 5 (Silver): Complete Crest Collector Level 5.
Crest Collector 10 (Silver): Complete Crest Collector Level 10.
Crest Collector 14 (Silver): Complete Crest Collector Level 14.
Adventure completed (Gold): Conclude the adventure.
Master Potioneer (Gold): Earn the Master Potioneer Badge.
Master Duellist Badge (Gold): Earned the Master Duellist Badge.
Master of Flying (Gold): Earn the Master of Flying Badge.
Platinum Trophy (): Earned all trophies.

Additionally, there are four secret trophies:

Mystery Solved (Bronze): Retrieve Slughorn's missing Memory.
Quidditch Champion (Bronze): Win the Quidditch Cup.
Escaped the Cave (Bronze): Escape the Cave.
The Burrow Defender (Bronze): Protect The Burrow.