Here is the full list of all 21 Plants vs. Zombies GOTY Edition achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
Grow the Tree of Wisdom to 100 feet
Discover the top secret zombie
Enable Mustache Mode
Collect all 49 plants
Get to 20 flags in Survival Endless
Complete a daytime level by planting only Mushrooms and Coffee Beans.
Get to a streak of 10 in I, Zombie Endless
Get the golden sunflower trophy
Get 8000 sun during a single level.
Defeat 2 Gargantuars with Corn Cob missiles in a single level.
Get to a streak of 15 in Vasebreaker Endless
Beat all 20 mini games.
Complete adventure mode
Defeat a normal roof level without using any catapult plants.
Bowl over 5 zombies with a single Wall-Nut.
Blow up 10 zombies with a single cherry bomb
Complete a daytime pool level without using pea shooters of any kind.
Hypnotize the lead dancer zombie
Pick up 30 coins in a row on a single level without letting any disappear.
Complete a nighttime level without planting any Mushrooms.
Blow up a zombie using a potato mine