Lisbon is all about steep cobbled hills, pastel buildings covered in hand-painted tiles, and endless plates of grilled sardines and pastel de nata. You want to go in the spring when the weather is warm enough for outdoor cafes but before the heavy summer crowds take over the viewpoints.
A typical trip runs about $2400 for flights and a place to stay. Spreading that cost out over time makes it easier to manage without stressing your monthly budget.
Remember that the intro window is a hard deadline, not free money. If you still carry a balance when those 18 months end, interest kicks in on whatever is left over.