Costa Rica in May or November puts you right in the green season, when the rainforests are dense and loud with life, and the crowds thin out. You get morning sun, afternoon showers, and better prices on places to stay. It is the best time to see sloths, monkeys, and cloud forests without fighting for a spot on the trail.
A typical trip runs about $3200 for flights, lodging, and getting around. Dropping that all at once hurts, so breaking it into chunks makes the trip actually doable.
That 0% intro window is a strict deadline, not free money, so make sure your automatic payments cover the $213.33 each month. If you leave any balance on the card when those 15 months run out, interest kicks in on the whole original amount.