Mass (or as some call it 'Weight')
Weighing is best to learn or consolidate practically so, if you can manage at home to get some practical experience of using and reading scales, it would be a great.
Talk about words such as heavier and lighter, and grams (g) and kilograms (kg). Discuss how there are 1000g in 1kg. Have a go at the investigation to estimate and weigh objects heavier or lighter than 1kg.
Why not have some fun baking cakes, buns or doing any cooking, and weighing out all the ingredients?