In a medium saucepan, combine water, vinegar, salt, and sugar.
Heat the mixture over medium-high heat, stirring until the salt and sugar are completely dissolved.
Remove the brine from heat and let it cool slightly.
Prepare two clean 16-ounce glass jars.
For the Classic Dill jar: Pack half of the cucumbers, 2 garlic cloves, dill sprigs, 1/2 tsp peppercorns, and mustard seeds.
For the Spicy Garlic jar: Pack remaining cucumbers, 2 garlic cloves, 1/2 tsp peppercorns, red pepper flakes, and sliced onions.
Pour the warm brine into each jar until cucumbers are fully submerged, leaving 1/2 inch headspace.
Tap jars to remove air bubbles and seal with lids.
Allow jars to cool to room temperature.
Refrigerate for at least 24 hours before serving.