In his eponymous Discovery Channel series, Stephen Hawking says that time travel is totally theoretically possible. Here are the three most straightforward ways to achieve it.
Build a wormhole, but through time, instead of space. Easy, right? But assuming wormhole construction is a possibility, if you placed both ends of the wormhole near Earth, and channeled through time, you'd arrive and a different when, rather than where.
Get in orbit around a black hole. Again, assuming you've got a spacecraft capable of interstellar travel, each revolution around a black hole would take only half as much time for those on the ship as those off of it.
Go really fast. Approach the speed of light — which would take a giant ship filled with massive amounts of fuel six years of full-burn to achieve — at which point time will slow down for the travelers on the ship. For the would-be chrononaut who can't find a black hole.