Simple syrup is nothing more than sugar dissolved in water. It's used instead of dry sugar to sweeten liquids because it disperses evenly (sugar alone tends to sink to the bottom of the glass). The flavorings below are starting points; feel free to create some of your own.
In a 2-qt. glass measuring cup, combine water and sugar. Microwave on high, stirring once, until sugar is dissolved, about 2 minutes. Chill until cold, about 1 hour.
Variations: When sugar is dissolved, stir in 1/2 cup lightly packed fresh herbs like basil leaves or tarragon, then chill until cold. When cold, strain into another glass measuring cup; discard herbs.