Many different educational websites have worksheets that contain conversion formulas from metric to customary units and back, with problems for students to solve. Generally, these sheets have directions that ask students to make specific conversions in each section, such as cups to liters or ounces to milliliters.
Games such as "Can You Fill It" on the PBSKids website challenge students to fill certain amounts of volume with specific containers without spilling anything. These games challenge children to multiply and divide effectively and use customary and metric units in solving problems.
Activities like "Liquid Melodies" from TLC Family take different liquid volumes in similarly sized glass containers and compare the sounds that those containers make when they are struck. Using a 2-liter jar, for example, and adding 1/2 liter, 1 liter, and 1 1/2 liters to them will make instruments with three different tones.
Different manufacturers make measuring cup sets that nest inside one another so students can learn the various liquid measurement units in customary and metric systems. These sets are available in customary units, metric units or both, so students know where the liter fits between the cup and the gallon.