Do you get a cup of coffee everyday? Eat out for lunch? Buy a candy bar and Coke in the afternoon? Find out how much your treat would be worth if you invested the total every month.
You could also find out how much interest grows over time depending on how much you put into savings as well as the length of time you can leave it.
Enter what you can save and the frequency you can save it. While you can't guarantee an interest rate, you can get a savings account at nearly 4.5% while mutual funds can average 15% a year.