London and Edinburgh make a great pair, especially in November or early spring. You get cozy pubs, fewer crowds at the museums, and crisp weather that makes walking between sights actually pleasant. A typical trip for two from the US runs about $3,900 for flights, hotels, and train tickets between the cities.
Instead of dropping nearly four grand all at once, you can break the total into steady monthly payments. With the right card strategy, you give yourself time to pay off the balance without interest eating into your budget.
Keep in mind that an intro window is a fixed deadline, not free money. If you still have a balance on your card when those 18 months end, standard interest rates will apply to whatever is left.