Liquid: Heat water in either a kettle, the microwave, or on the stove until steaming hot. Stir in sugar and salt until dissolved. Stir in vinegar.
Jalapeños: Thinly slice the jalapeños (¼ to ⅛ inch thick), optionally removing the seeds for a less spicy pickle.
Combine: Add your chosen optional flavors to a lidded non-reactive container (like a glass jar or ceramic vessel - I used a 16-oz mason jar), then top with jalapeños. Pour vinegar mixture over jalapeños so that they are covered (you may not need all of the liquid). Let cool to room temperature, then seal shut and transfer to the refrigerator. (Let pickle for at least 30 minutes before digging in.)
*Make it less spicy by removing the seeds from the jalapeños before pickling.
Store in the fridge for up to 2 to 3 weeks. Serve these pickled jalapeños on salads, nachos, and tacos!