The most flexibility you will get with a regular credit card that allows earning miles rewards which can be redeemed with any airline. For example, the Discover it Miles card will allow you to earn miles on everyday purchases and redeem them with any airline of your choice without blackout dates or limitations. The card will give 1.5 miles per dollar spent on all purchases. This means that you will earn miles even when you shop at local supermarket, dine out, or book a flight. The earned miles can be easily redeemed for travel expenses that appear on your credit account.