Hi Jacey, glad you enjoyed Japan so much too! I can't wait to make it back there and explore some more.
I have traveled quite a bit around South America, and if it's your first time, I would highly recommend Chile, I feel it is the most accessible South American country to visit. It's a bit like California, with as much geographic diversity. With one week, you can set up nicely in the capital Santiago, in the shadow of the Andes. From there you can tour local wineries in the Maipo Valley, day trip to Portillo in the Andes, or make a relaxing side trip to Viña del Mar, on the Pacific.
For more adventure your could instead visit the Patagonia/Tierra del Fuego area, aka the most southern part of the world. The town of Ushuaia, in Argentina, makes for a splendid base to explore this area. There are direct flights from Buenos Aires to Ushuaia, which takes about 3 hrs. This is the next level up from Chile, in terms of difficulty.
Also one more thing on South America, because it's in the southern hemisphere, remember that their seasons are flipped, so January is actually in the middle of their summer, just something to keep in mind :)