The Demilitarized Zone (DMZ) is the 4km-wide buffer strip running across the Korean peninsula along the armistice line, and a day trip from Seoul to see it is one of the stranger travel experiences available anywhere. Tours depart from central Seoul in the morning and can include Panmunjom (the Joint Security Area where armistice talks happened), the Third Infiltration Tunnel (one of four tunnels dug beneath the border by North Korea), Dora Observatory (from which you can look into North Korea), and Dorasan Station (the last station in South Korea before the border).
Multiple tour companies offer DMZ day trips from about $50-100 USD. The USO runs its own tours. Independent access to most of the key sites is not possible, so a tour is generally necessary. The experience is sobering rather than thrilling — the landscape is eerily quiet, the security is real, and the political situation is not an abstraction here.