You, sir, are very uncreative.
If I had a time-stopping watch (a GOLD time-stopping watch, no less), I’d much rather walk into people’s showers, bank vaults, convenience stores, nuclear silos, etc. But, of course, the possibilities of having a time-stopping gold watch would be a thread all by itself.
I’d like to write a virus that changes the little message you get when you shut down Windows from “It is now safe to turn off your computer” to “It is not safe to turn off your computer.”
Okay, most people wouldn’t notice…but when they did… 
How about getting some ink that changes colors in temperature (remember those color changing shirts of 13 years ago?) and write naughty things on shirts in stores in the invisible ink. And then when they wear them outside stuff appears on their backs, heeheehee.
double stick tape seems like there are loads of fun stuff you could do with it. That and vaseline, giggle snort hmmm, oh the possibilities… Too bad I’m such a coward, sigh.
Just the other week I was in a bookstore with my girlfriend, and we noticed they had the book “Healing Homosexuality”. After scoffing at it for several minutes, my muse paid me a visit: I went over to the humour section, picked out “You Know You’re Gay When… : Those Unforgettable Moments That Make Us Who We Are”, and delicately placed it right next to the first book. Then we scurried off.
In hindsight, I really wish I’d put Eminem’s biography next to it. Oh well.
Oh, and jr8, it’s very simple to edit the shut down screen. It’s just a file in your Windows directory somewhere, called “logo.sys”, and you can edit it with most image editing software. Subtle changes are good. For example, keep the default font and colour, but change the message to read “I die a little every time you turn me off”, or “It is now safe to get naked and scream.”
Not really product messing, but I’ve always wanted to pass out tampons instead of candy on Halloween…
Also, I remember when I worked at a grocery store, we would steal those little orange stickers from the meat department and put them on our smocks, that said things like Sweet, Hot and Spicy, etc etc. I found one on a package of bread, that I was tempted to use: Try Me! Only a 1.49!