With due respect to Deadly Nightlight from this other thread,
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Think of all the fun pranks you could do!
Set some under a couch cushion ala Whoopie Cushion when your Aunt Edna comes over for a visit. She sits down and… FWOOMP… no more grouchy Aunt Edna!
Sprinkle a little in your kitchen sink and … FWOOMP… No more annoying dirty dishes!
Or you could get the hell away from it. It reacts violently with matter and generates gamma rays that penetrate anything and rearrange you at a level you don’t want messed with. If a gram of antimatter appeared within earshot, you’d regret it greviously. Better to be a slug and find yourself in a rocksalt warehouse.
Alright, Napier. Let’s pretend we can defy the laws of physics.
If I had some antimatter, I’d fling it at people like the monkeys at the zoo do with their crap. Oh man, we can learn so much from those monkeys.
Mail it to my first ex? Of course, I have no idea what happens when antimatter gets with range of a Black Hole. 
Where’s Achernar? He might know.
I’d make a baseball out of it and become a major league baseball pitcher. I would lob easy hits to some big homerun superstar and they would swing for the moon to knock it out of the park. Jokes on them though, because their bat would disintegrate as soon as they hit it.
Drop it off a bridge onto the cars below. There’s simple elegance there.
Oh, I’d just keep it in my pocket.
Um, wait- that’s not right.