Melon Playground is a casual video game in the ragdoll/physics sandbox subgenre where you have an open space where you can place various living creatures and mistreat them however you want.
Gameplay in Melon Playground begins with choosing the type of map you want to play on. After that, you can add various elements to the map from the dropdown menu on the left. For example, there are creatures made of fruit who look like people and can move, suffer, and die (but you don't have to feel bad, because they're just watermelon- and melon-people). There are also all kinds of weapons and objects you can use against them.
You can inject their bodies with acid, shoot them, hit them with different objects, revive them with another type of injection, monitor their vital signs, set off a bomb, try to put them back together, put helmets on them, put armor on them, see how they fare against various attacks, and anything else you can think of. When you get tired, you can restart the scenario or create a new one.
Melon Playground is a game where you can experiment with physics to see how characters respond to different objects. Basically, you can do whatever you want, which is the essence of any sandbox (although in this case, the aim is to cause harm to fictional creatures).