Do you have a little garden in your school?
So why don't allow your pupils to grow flowers and vegetables in it?
Even if you don't have space, and the only area you can use is the windowsill, few pots and a selection of seeds might improve the enthusiasm of those young learners.
Plants like radishes and lettuce grow very fast. Your students will be able to collect them after few weeks, with great satisfaction.
Slower plants like charts, strawberries or tomatoes, otherwise, can teach them that there is the right time for everything.
Decorate the pots with labels to remember which plant is in it and how to take care of its life. It could be a craft activity to teach them new vocabulary.
You can also teach 'responsibility' handing tasks out to each one (or to each group if they are very young).