These days, everybody has a gimmick - it's how you get attention. Sometimes that gimmick is something simple, like low prices or excellent service. Sometimes it's a giant pink bear dancing in front of your business. Whatever it is, it needs to answer the question: why should people give you their business?
For a lemonade stand, this could be as simple as a big sign advertising lemonade, but it could also be more creative. You could use food coloring, for example, and offer "green" lemonade, or draw a graph showing how close you are to a fundraising goal (which could be simple, like "$10 in sales", or more complicated like "$92.75 for a new bike"). Again, the point is to get people interested in what you're doing, which makes them more likely to hand over their money.
Whatever you do, remember to keep track of your spending; you want to know exactly how much money you have going in and out so you know how much you made and whether you had a sufficient profit for the time investment to make it worth doing this again. Additionally, tracking your profit over multiple occasions allows you to see what works and what doesn't.
Above all, have fun!