A beet salad with feta and walnuts can be made a few hours in advance and stored covered in the fridge, but don't add the dressing until just before serving. Leftovers can be stored in the fridge covered well or in an airtight container for up to a day. The arugula will wilt but the salad will still taste good for lunch, for instance, with pulses or grains or brown rice stirred through it.