Make sure to peel and cut your fruit into pieces that your blender can manage. Your fruit can be frozen or fresh depending on your preference. Some healthy fruits that you can use include, banana, watermelon, mango, and peaches. Agave nectar is a healthier alternative to sugar and will help enhance the taste of your Slurpee. If you can't get Agave nectar, you can use lemonade or more sparkling water as an alternative. If you like your Slurpee sweeter, then add more agave nectar. Blend all of the ingredients together until your drink reaches the desired consistency. Mixing the Slushee for a longer period will result in your slushy being more watery than the traditional Slurpee found at stores. Drink the Slurpee as soon as you're finished blending, or the ice will melt, and it will lose its consistency.

Summary:
Add fruit and water into your blender. Add agave nectar or lemonade and ice to your blender. Turn your blender on high until your drink is slushy.