You definitely need an app to plan your daily tasks, and, at the moment cheapest one is our 🙂 only for $0.99.
However, outside of ad-jokes, if you don't know that you need an application for doing something, better don't spend money on random apps. Money on giftcards will not disappear with time, so you should wait when you need 'that app'.
At least, you would be able to spend them on Lion upgrade, or other major applications, which are really required.