Minecraft?

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Let's see. There's nothing not-fun about it. It can't get boring.

You have the option of your difficulty, if you want no monsters spawning, you should probably play on the difficulty of Peaceful. Now, you'll start out lagging if you don't have a good computer. You'll want to press ESC -> Options -> Video Settings -> Graphics: Fancy. Change that to Fast. Then change Render Distance to a distance that won't lag you.

It is fun because you can build whatever you want, craft really cool things, and survive. I can't explain, but you can do whatever you want. Terrain generation never ends.