Instead of: "many," "a lot," and/or "several" use: 

Plethora, Myriad, Innumerable

Instead of: "big," "tall," and/or "huge" use: 

Towering (usually to describe a person's height), Vast (usually in regard to the size of an area).

(Truthfully, when writing an academic essay it's better to give an exact or approximate height/size to the reader.)

Instead of: "small," "tiny," and/or "little" use: 

Minute (My-noot)

Instead of: "calm," "peaceful," and/or "expressionless" use: 

Stoic (suggests an ability to withstand great pain or misfortune), Placid

Instead of: "rude," "unkind," and/or "disrespectful" use: 

Irreverent, Belittling, Disparaging

Instead of: "amazing," "wonderful," and/or "cool" use: 

Astonishing, Spectacular, Phenomenal

(These recommended words aren't anything special, [ironic, huh?] but the words they're to replace are simply overused.)

Instead of: "kind," "easy-going," and/or "friendly" use: 

Charismatic, Gregarious, Personable, Affable

Instead of: "interesting" use: 

Dynamic, Complex, Enthralling, Enraptured

Instead of: "important" use: 

Notable, Paramount, Vital, Venerable (technically means deeply respected)

15 Bonus Words for Creative Writers: Instead of: "said" use: 

Exclaimed, Chortled, Croaked, Bellowed, Murmured, Sobbed, Declared, Corrected, Elaborated, Snapped, Barked, Enlightened, Breathed, Predicted, Beamed.

(These all hold a specific expression behind them, which, does mean you'll have to google the meaning if you're unfamiliar with the word, but it truly does take your writing to the next level and helps make your character(s) feel more life-like.)