Online Activities for 3rd - 6th Classes
Khan Academy - Mass and Volume (Third Grade): Watch the videos and then answer the practice questions. Afterwards, for something more challenging, look at the Fourth Grade activities on Estimating Mass and/or Converting Units of Mass. Or even the Fifth Grade activities on Metric Units of Mass. You can also register for a free Khan Academy account to record your progress and explore other topics/grades.
Measurement: a selection of games from ixl.com. Choose the games to do with mass/metric units of mass and mixed metric units. You can do a number of free quizzes each day without having a subscription.