I've been on a nice pickling run lately with red onions, carrots, and I recently added grapes to the mix. I originally tried them in a farro risotto at Perilla and was blown away by the sweet and sour taste. I came up with the recipe below on my own, but I'll try it next with a sprig of rosemary, ginger, brown sugar, and/or peppercorns.
1/2 cup - White Balsamic Vinegar (or White Wine Vinegar)
1/2 cup - Sugar
1 - Cinnamon Stick
1 cup - Red Grapes (or enough to fit a 8oz Ball Mason Jar)
1) Rinse, dry, and slice the nipple part of the grape off.
2) Put the grapes into a mason jar.
3) In a small saucepan, combine the vinegar, sugar, and cinnamon stick and bring to a boil.
4) Pour the liquid over the grapes, close the jar, and put it in the fridge.