Honolulu warms up in the spring, but the summer crowds haven't arrived yet. Mid-April through June is the sweet spot for Oahu. You get clear skies on the North Shore, shorter lines in Waikiki, and better prices on places to stay than during the winter rush.
A solid week on the island usually costs about $3,500 for two people once you cover flights, a place to stay, and a car. If you don't want to drop that cash all at once, you can spread the cost out over time without paying extra fees.
Remember that a 0% intro APR is a deadline, not free money. If you still have a balance when the 15 months are up, regular interest rates apply to whatever you haven't paid off.