Combine 1 part apple cider vinegar with 1 part water and add about 1/4 cup sugar per cup of vinegar. Stir until dissolved and add salt and pepper as desired. Slice 1/2 sweet onion and add to the vinegar solution. This will cover about 2 cucumbers, peeled and sliced. For the best flavor, refrigerate this at least one day before eating, but it tastes pretty good right away as well.
These last for quite a while in the fridge, so it's a good way to use up those cucumbers that you don't think you'll get to before they go bad.