Make the rhubarb simple syrup. To do this, add the water, chopped rhubarb and sugar to a medium saucepan and bring to a boil stirring occasionally. Once it's hit a boil, reduce heat and simmer for about 20 minutes. Remove from heat and let cool before straining. You'll have extra, and it'll keep in an airtight container in the fridge for a couple of weeks!
Now make the margaritas. To do this, add the tequila, cointreau, lime juice, strawberries, ice, and 1/4 cup of rhubarb simple syrup (or more, if you'd like a sweeter drink) to a blender. Blend until smooth and enjoy!