Many modern freezers have automatic ice-cube makers, but trays are far from obsolete. Use the standard segmented kind to freeze the following (then move the cubes to freezer bags so they don't absorb odors or flavors):

Small quantities of foods you often cook with — chicken or beef stock, tomato paste, the last of the red wine. You'll be able to use a cube as needed, rather than having to open (or defrost) a whole container.

Cubes of leftover coffee or tea. Use them, instead of watery, diluting ice, to chill their liquid counterparts.

Single servings of marinara sauce or pesto to toss onto a plate of pasta.

Leftover egg whites. Freeze one egg white per segment, and use as needed for baking. (Frozen egg whites will keep for about a year.)