All monsters have a chance to spawn with a mark on them. You get a box for each marked monster you kill and these boxes change according to the level.

Boxes

Beginner Box(Lvl 1-50):
Shout of Elemental
Elemental Stones (Lv2-3-4)
Random Prayer Stones (Lv1-2)
Lens Stone+
Random Fireworks
BBQ of Acclimatization
Soup of Attack
Broiled Meat of Vitality
Rejuvenation Roast
Rejuvenation Pie
Sugary Sweet Potato
Reinforcement Potion
Extra-large Light Speed Potion
Extra-large Quickness Potion
Extra-large Defense Potion
Extra-large Giant Potion
Extra-large Evasion Potion

Intermediate Box(Lvl 51-70):
Fruit Ice
Lucky Charm
Rick Man's Scroll
Shout of Elemental
Lens Stone+
Compound Potion
Elemental Stones (Lv4-5)
Prayer Stones (Lv3)
BBQ of Sage
Soup of Raid
Broiled Meat of Life
Healing Roast
Healing Pie
Honey Sweet Potato
Reinforcement Potion
Fantastic Emerald
Fantastic Zircon
Fantastic White Pearl
Fantastic Amethyst
Serazaran Powder
Santa's Bell

Advanced Box(Lvl 71-92):
Fruit Ice
Lucky Charm
Rick Man's Scroll
Shout of Elemental
Kitty Lv1 1 day
Lens Stone+
Compound Potion
Extra-large Mana
Random Prayer Stones (Lv4)
BBQ of Nirvana
Soup of Fury
Broiled Meat of Transcendence
Health Roast
Health Pie
Syrup Sweet Potato
Extra-large Light Speed Potion+
Extra-large Quickness Potion+
Extra-large Defense Potion+
Extra-large Giant Potion+
Fantastic Emerald
Fantastic Zircon
Fantastic White Pearl
Fantastic Amethyst
Serazaran Powder
Santa's Bell

Expert Box(Lvl +92):
Silver coin of the rich
Lucky Charm
Rick Man's Scroll
Shout of Elemental
Kitty Lv2 1 day
Lens Stone+
Compound Potion
Extra-large Mana
Seed of Farmer
BBQ of Nirvana
Soup of Fury
Broiled Meat of Transcendence
Health Roast
Health Pie
Syrup Sweet Potato
Platinum treasure chest
Platinum Key
Extra-large Light Speed Potion+
Extra-large Quickness Potion+
Extra-large Defense Potion+
Extra-large Giant Potion+
Serazaran Powder
Santa's Bell