To leave one stall, in a public restroom, without paper, for one with.

A total waste of the machine, as you are being governed by the moment, rather than the majesty of knowledge and experience that would await you, if you were not.

Hey guys what does this button do?

Mmmmh, all things considered, I could definitely take another shot at my end-of-the-year meals.

Going back in time and stopping yourself from going back time.

I'd go back to early July 1947 in Roswell, New Mexico. There I would get the Army public information officer really drunk and make him issue a press release about a flying disk crash.

I believe in life on other planets with every ounce of my being, but that was a balloon.

You'll get no arguments from me. But the guy issuing a press release that it was a flying disk is so out there that he had to be drunk.

A determinedly useless effort with a time machine would be - to go back and find out which cat did the thing they are fighting about so I can explain it to them when I get back.

Other fun, but useless ideas - tour major cities/destinations in 10-50 year intervals going backwards, criticizing all the changes they are going to make in the future to anyone you meet.

Offer sales coupons to businesses that don't exist yet, telling the recipients to hang on to these silly bits of paper because they will be worth something someday.

Photobomb press photographers, premiere openings, and other major events of the past wearing/using obvious clothing / technology from the present and future. Come back and laugh at the ways all your appearances are explained away...

Going back in time so you can post in a forum thread while it's still active, so as not to have to engage in the black art of thread necromancy.

Replace a picture of a knot with a 403 page? =p

Mine works... I'll repost it here

Aaahh, that's better.
...And yes, that would indeed be lame use for a time machine. =)

Although, now that I've thought on it, messing with the printing of a book in the 1950s or earlier -- say, replacing a picture of a square knot (or of anything, really) with a picture of a 403 (or perhaps 404) error -- would be both brilliant, and a horrible misuse of time travel technology.

Weell. Going back and forth in time - checking the winning lottery numbers - then going a day back in time to put those in - so many times I'd have gazillion billion dollars and then I'd go back in time one more time. I'd give it all to Double Fine if it existed in then, but make them promise to buy Lucasfilm from George Lucas a week before Disney did :p
Then just come back to present and have myself a barrel of beer :)
edit: and the next day with a throbbing headache realize I don't have any money to buy any of the Double Fine version games of Lucasarts remakes :p

#1. to go back and make myself pick a university degree that would've actually gotten me a job
#2. to go back and sleep with that girl I rejected for some goddamn stupid reason...I'm a moron, whats wrong with me...