Inside of a customer's account, allow them to "buy" store credit, and use a cash to credit multiplier similar to what we do on the card buy list. This multiplier should be independent of the buy list multiplier however.
Then in your web site you should be able to have anyone buy store credit for another user by providing that user's email address and a date when the store credit should show up in their account. This would allow people to buy gift certificates for birthdays and other special events without having to come into your store.
We have many kids that play at our store, and their parents often "buy" their kids credit before Friday. Currently, we do it as a giftcard sale, and then add credit instead of handing over a card. Gift cards are non taxable. This method doubles your revenue for that amount, however, on all the reports, which is not what you want (a 20 dollar gift card adds 20 dollars of revenue, and then the sales from the credit add another 20 dollars. On paper, it seems like you've sold 40 dollars).
We've had customers asking about buying store credit so they can "save up" for products that are higher priced. We have also been asked about gift certificates and gift cards. I'd also like to be able to do credit not just cash for this. Some shops have minimum charge limits and have customers purchase a minimum of $5 or $10 store credit to minimize multiple small credit card purchases by having people purchase a chunk of credit during events and then they don't get wheedled by the per item card fees for 50 cent chips or 75 cent drinks.