Greece in late spring or early autumn is hard to beat. Skip July and August when the heat and cruise crowds peak, and go in May, June, or September instead. You still get warm Aegean water, open tavernas, and ferry runs between islands like Naxos and Milos, but at lower prices and with breathing room.
A realistic two-week island trip runs about $4200 for flights, stays, and ferries. If you do not want to pull that cash out of savings all at once, you can stretch the cost over time without paying interest if you plan it right.
A 0% intro period gives you breathing room, not free money. If you still have a balance on your Citi Simplicity® Credit Card after those 18 months end, the regular interest rate kicks in on whatever is left, which gets expensive fast. Set up automatic payments for $233.33 each month so you hit zero right on schedule.