I'd personally do a gas card (credit. chevron, texaco, shell, etc) and just set the limit low and be certain the you can pay that entire limit in full every month. Doing that as opposed to doing something like a gift card is that you can build your credit score and credit history without spending any more than you already would each month. If you pay it in full every month you don't run up any interest either. I've had a chevron card since I was 18 (25 now). Easy way to establish or improve credit. You get used to only being able to use one gas station. Just pick one that has at least one station on your most common commute. It will also keep you from hitting up the cheapo stations that will only cause nasty buildup in your feul system anyway.
Perhaps you can deposit money onto the card (to make it like a perpaid card) or overpay your first month to establish a positive balance then adjust the avaliable credit down to the amount of a single tank of gas. That way you really only using the case you deposited to the card (pre-paid you could call it I guess) and just in case you oops and loose track you have enough avaliable credit on the card for one "oops" tank of gas.
