Of course! Simply make your brown sugar syrup the day before, let it cool completely, cover it tightly with plastic wrap or in a mason jar, and refrigerate overnight. You can also prep your peaches the day before by slicing them and squeezing a bit of lemon juice atop of them to avoid them from browning---then store in an airtight container and refrigerate. The best part about making these ahead of time is that they'll be completely chilled once ready to serve. Also, the ice cubes will be made in advance (crushed or whole) and ready to add to your assembled mocktails!