Although this has already been taken, I would invent a machine capable of traveling to anywhere in space and time with a way to preview where you would be going, so that you wouldn't kill yourself by accidentally going into lava or a supernova or something.
time glasses. you can see what happened in the past or what will happen in the future.
a healing machine. if you have aids or diarrhea or you loosed an arm it heals you by extracting the "bad" cells/bacteria/virus and re-create it. it can train your immune system so you cant get aids (or any other diseases never ever again. your sleeping in it and when your awake you have a new arm and can move it as like you have never lost it! the only problem is that the machine need living cells to create out them the same human (and i mean NOT cloning).
a machine that can create any meal if you say it. you must only say what you want, how much you want and how you want it (example: you stand before the machine and say "i want three well done grilled steaks" then you get three well done grilled steaks).
How about a machine that can turn any food into grandmas peanut-butter-fudge,and you get the health benefit of the original food.(you can also use trash so its good for the environment)
I would invent a mind controlling device that could change people's opinions thoughts, reasoning etc. Also every time you would use it you would live another year...
portable hologram of a full sized NBA basketball court that i cud carry wherever i go and play wherever im at. And id invent a portable collapsable ball so im ready to play anywhere
A solar-powered flying machine that transforms into a wristwatch. I can go anywhere I want, whenever I want and I don't even have to pay a penny for gas.