Two words: minecraft IRL
In all seriousness though, I'd probably think it was fun after getting over losing everyone I knew. I mean, I'd definitely go insane and talk to myself or made up people endlessly, but I probably woundnt have a total mental shutdown and off myself. I'd really just want to do a few things. Mainly,
1. Learn to become self-sustainable. Food goes bad, no getting around that.
2. Build some kind of fort.
3. Probably at some point go on a destructive rampage.
4. Explore.
5. Learn parkour. No one's around to tell me to get off that.
6. The last thing is something that one of my teachers said he'd do when asked the exact same question: Record the knowledge of man in something permanent, like carved into a mountain. At some point, you're going to die, and all major signs of humanity will eventually disappear. In the off chance that a sentient alien race ends up finding earth, there will be something to knwo that we were here and we figured at least some stuff out.