In a high speed blender, pulse all ingredients together. Be sure not to over blend as that will create a smoothie rather than a slushie. Enjoy right away otherwise the slush will melt and it will become watery.
Note: these measurements can all be adjusted to your liking. To make it sweeter, add more maple syrup. To make it more tart, add extra lemons. To make it more slush-like, add more ice.