These make a terrific food-themed gift, but make a batch for your own tree, too. All manner of citrus fruits work well, including clementines, meyer lemons and tangerines, so use whatever you have on hand. Since these ornaments are a cinch to make, it's best to compost them at the end of each season and make a new batch for the next holiday, instead of trying to store them throughout the year.
Preheat the oven to 200°F. Line two large baking sheets with parchment paper; set aside.
Using a serrated knife, trim off and discard the ends from each lemon, lime, grapefruit and orange. Cut each piece of fruit crosswise into slices, making each as thin as possible; discard any seeds as you go.
Arrange sliced fruit on prepared baking sheets, spacing them about 1/2-inch apart. Bake, rotating baking sheets and flipping fruit about every 30 minutes, until just dried out, 2 to 3 hours. (Since the thickness of the slices may vary along with the sugar content in each fruit, remove any dried slices as they finish, returning the sheet tray to the oven to let others finish baking.)
Set ornaments aside to let cool completely, then pierce with thin ornaments hooks or paper clips for hanging.