Fold paper towels or napkins into large and small fractions. Start with halves and move to eighths and sixteenths. Use magic markers to label the fractions. Then show children how to use a scale to weigh the objects. Next, have them estimate their own weight, as well as that of other family members, and use the scale to check their guesses.
Some brave parents get on the scale, too.
Add water to an ice cube tray and set it in the freezer. Ask your child how long it will take to freeze. For variety, use different levels of watering different sections of the tray. Set ice cubes on a table. Ask your child how long they will take to melt.