LEVELS

Garden Levels
Complete the 42 garden levels by harvesting the required plants.
Harvesting all of the 'level-up' seeds unlocks the Bonus Seeds (currently just Bamboo to attract the Panda).
 
Wildlife Levels
Earn a set amount of Diamonds to reach the next Wildlife Level.  Upon reaching Legendary this unlocks the Hidden Bonus Levels and each new Legendary Level unlocked enables the use of the next Mushroom to attract the next Unicorn when Ready to Harvest.
 

WILDLIFE

Rainforest
Complete task requirements in order to unlock the next Jewel seed.
Every 10th you spot in other gardens drops a specific Enchanted Jewel seed for use with the Endangered Wildlife and Hummingirds.
 
Hummingbirds
A complete nest built from 5 different Hummingbirds will be ‘Indestructible’ and will never need repairing.
 
Mystical Butterflies
Complete this section to unlock the ability to purchase the Supershroom with 10 Forest Ruby seeds.
 
Fairytales (Reward - Gnome Table)
Complete all of the Fairytales to unlock the Gnome Table for use as a 3rd table in your garden.
 
Rainbow of Fish (Reward - Rainbow Rocks)
Completing this section of the Pond Wildlife will unlock the Rainbow Rocks which makes any Mushroom planted in them, Ready to Harvest immediately.
To attempt this section you must be at least Legendary Wildlife Level.
 
Bottom Dwellers (Reward - Birdbath)
Completing this section of the Pond Wildlife will unlock the Birdbath which attracts the adult birds of the Baby Chicks aswell as the Baby Chicks, but only the ones which have previously been hatched in your garden via the normal nest/egg method.  All Baby Chicks spotted in the Bird Bath will also drop Gold for the Spotter and the Garden Owner.
To attempt this section you must be at Legendary Wildlife Level, have completed the Rainbow of Fish section and be able to get Frogspawn in your garden (see the instructions in the wildlife guide under the Frog Breeding section for more info)
 
Topiary Trails (Reward - Greenhouse)
Successfully harvest all of the Topiary Trails required to unlock the use of a Greenhouse on which you can store 3 plants.  (Those plants which would enable ‘cheating’ i.e Birthflowers among others, cannot be stored in the Greenhouse).
To complete this you will need to be Legendary VII Wildlife Level as you will need to have unlocked the Rainbow Mushroom and  also, at least Garden Level 37 to have access to the Aureus Naturalis seed.

Dinosaurs (Rewards - Free Superfood for 48hours)
Unlocked after you complete the Dino Challenge by attracting 40 of each Dinosaur!
You need to spot Dinosaurs to build nests on the Nesting Rocks (gift item). Each nest can only be built by the same dinosaur.
Your reward for completing the Dino challenge is a Free Superfood Weekend (48 hours) every month, which you can start whenever you like!

Alice (Reward - Mat Hat Table)
Unlocked after spotting and attracting all the Alice wildlife as stated in the game details.

Chameleons (Reward - Chamelepot)
After completing this challenge (by both attracting and spotting the Rainbow Chameleon at least once), a special Pot will be unlocked for you in the 'Misc' section of the Pot Shoppe - this pot allows you to instantly grow a Rainbow Chameleana flower any time you wish.
 
QUESTS (Attracting Extinct Wildlife)
Completing Quests and earning scrolls unlocks a section on one of two Poles for painting.  These painted sections then attract certain Extinct Wildlife.

TABLE PARTS
Complete Level VIII (collect 420000 scrolls) unlocks the first 12 parts
Complete every quest at every level (except Seasonals) will unlock 3 more Tops, 6 Middles  and 1 Bottom (mini flowers/mushrooms surrounding the table)

SEASONAL MINI CHALLENGES
 
Sunflower Growing
Growing Sunflowers to certain heights by spotting and attracting Summer Wildlife (as specified in the release notes on the News Page) rewards vary.
 
Pumpkin Growing
Growing Pumpkins to certain weights by spotting and attracting Halloween Wildlife (as specified in the release notes on the News Page) rewards vary.
Growing a Pumpkin to over 30lbs also produces a Halloween background effect which lasts for as long as he Pumpkin remains in your garden.
 
Holiday Calendar (Rewards Vary)
Open the door and spot the wildlife shown to earn that days reward. The door remains open for 12hours and you can then open the door for the next day. You can only open doors at a maximum of 2 days ahead of the actual date.
 

BUILDABLE ITEMS

Nests – Complete nests will host an egg
Festive Trees – Completed Tree’s attract Rudolph as well as the Winter Wildlife.  Produce the Flying Santa, Trees  & Falling Snow background effect.
Snowmen – Completed Snowmen attract the Polar Bear and the Festive Winter wildlife and produce the Icicle garden effect.
Santa Snowman – Completed Santa Snowmen attract the Polar Bear and the Elf Winter Wildlife and produce the Icicle garden effect
Scarecrow – Completed Scarecrows attract the Halloween Wildlife and the Ghostly Wildlife and produce the Flying Bats garden effect.
Witch Scarecrow - Completed Witch Scarecrows attract the Halloween Wildlife and the Zombie  Wildlife and produce the Flying Bats garden effect.

Thanks for all the info, Amanda!

This is definitely the thread to read as a refresher as well as the new garden phase faeries. Thanks for the facts.

LEVELS...edit...
 
Garden Levels
Complete the 42 garden levels by harvesting the required plants.
Harvesting all of the 'level-up' seeds unlocks the Bonus Seeds (currently just Bamboo to attract the Panda, Red Panda & Gorilla).
 
Wildlife Levels
Earn a set amount of Diamonds to reach the next Wildlife Level.  Upon reaching Legendary this unlocks the Hidden Bonus Levels and each new Legendary Level unlocked enables the use of the next Mushroom to attract the next Unicorn when Ready to Harvest.
 
 

Maybe just add that the Mad Hat Table can be raised and lowered at will, with diamond dust.

Love this!  Thanks so much, Amanda, for taking the time to post it.