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