20 light years from our own solar system is a red dwarf star, Gliese 581. Astronomers have confirmed the existence of 3 planets around it. The tradition of naming for planets around stars is to name them with the star's name followed by a letter, starting with 'b' for the first planet. (Because the star is 'a.')
Gliese 581b seems to be a gas giant around the size of Neptune. Gliese 581c seems to be similar to our planet Venus, but larger. Gliese 581d is in the habitable zone, and is speculated to be similar to earth, with liquid water and more. All three are likely to be tidally locked due to their proximity to their home star, meaning that they have a day side that always faces the star and is lit constantly, and a night side facing away from the star that is dark forever.
I think we should send a probe to Gliese 581d, which would broadcast video of circulating the planet when it arrived. It should then land, construct an airtight greenhouse, fill a tank with the local water, and plant the seeds of earth plants in the greenhouse. In the remote chance that Gliese 581d has native aliens, they would probably find this interesting (organisms from ANOTHER WORLD, sealed off so it won't harm us, holy crap!), and if they don't, then we've kick started terraforming the place in case something happens to earth. (Probably the greenhouse would, at some point, leak, and carbon dioxide and methane are gases likely to be common on other planets that plants can use)
Now since Gliese is 20 light years away, and the absolute maximum speed we could manage is about .8c, if we launched this probe tomorrow at top speed, the probe would land 25 years from now, and we'd get the video about it radio'd to us in 45 years. Also, this would cost multiple millions of dollars. Still, awesome, right?