Getting Mjolnir* (and being able to use it) would be one of the coolest things ever. Turning into a god, flying, smashing stuff, controlling storms - it would be so much fun! Sadly though, I doubt I'll ever get it. Not because it's fictional or anything like that, but because I know right now that I would be really irresponsible with it. I wouldn't kill anyone, and I doubt I would even seriously injure anyone, but I can easily see myself flying around and destroying rocks and stuff, or making it rain on people I don't like.
*Shame on you if you didn't know that Mjolnir is Thor's hammer!