Many Delta credit card signups include a $50 or $100 statement credit if a Delta purchase is made within the next three months. For many, however, a Delta purchase is not immediately foreseen and eventually that statement credit is left un-utilized. One easy thing to use the statement credit on is a Delta e-Gift Card. The same basic principle applies as is used when using up the annual $200 travel credit from the American Express Platinum Card. That article is available here.
Go to the Delta eGift Card page here. Then, set the certificate amount to $50, and the amount to 2. This is the only formula that I have tested and verified to work so far, but one $100 gift card should hypothetically work too.
The page should look like this:
Submit the form, and within two days the transactions should post and be credited back to your account.
Remember that only three gift cards can be used per purchase on Delta when using the gift cards. This slightly limits redemption, but the gift cards never expire.
With this method, you can always extract the full possible signup bonus from Delta and American Express.