May and September are the sweet spots for the Cyclades. The summer crowds clear out, the Aegean sea stays warm, and ferry schedules run full steam between Santorini, Naxos, and Milos.
A solid two-week trip from the US runs about $3400 all-in. Instead of draining your savings account upfront, you can lock in your dates and pay the trip off in predictable monthly chunks before you step on the ferry.
Keep in mind that 0% interest is a deadline, not free money. If you still have a balance when the 18 months end, the card's standard variable rate kicks in on the remaining amount.