Here is the full list of all 8 Garden Simulator Demo achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Throw trash into the garbage container from a significant distance.
Walk on your rooftop.
Give your cat some water.
Play for one whole week.
Mow half a square kilometer.
Own 50 garden coins.
Own 250 garden coins.
Own 500 garden coins.