New Zealand in December through March is summer, and it shows. Long golden evenings, beaches you can actually swim, glaciers that look bluer in the sun, and trails like the Tongariro Alpine Crossing or the Routeburn without winter snow blocking the tops. Most of the country is easy to drive in a couple of weeks, and the South Island tends to be where people fall hardest for the place. The catch is that December and January are peak season, so flights and the better lodges fill early, sometimes two or three months out.
For a roughly $1,250 trip from the US, the big variable is airfare. Once you are on the ground, food, a rental car, and simple stays can stay reasonable if you skip the resort-tier options. The aim is to plan the trip first, then figure out a clean way to pay for it without sweating the bill.
The 18-month intro period is a deadline, not a free trip. The monthly math of about $69.44 only holds if the balance is paid down or paid off inside that window. Once the intro ends, any amount still on the card starts accruing interest at the regular rate, which can add up fast on a $1,250 balance.