What are some fun monsoon activities for kids?

The first I would say is that let them get drenched in the monsoon shower. And rest, you can put them in any activity you think your kid has interest in. Playing football in the rain would be quite an experience for your kid. You can also help them plant some trees, or flower seeds. Monsoon is a good time to sow the plants. So, let them have fun and at the same time help in making the surroundings green. You can make paper boats with them and see them go in a puddle. Let them in the mud and get them the whole feeling of being close to nature. Let them be messy and muddy, sometimes you should let them be. After they are done, you can always turn them into a prince or a princess, you know that you are a pro in that. ;p