Teach Climate has some good ideas including engaging videos.
The NASA website has loads of practical ideas, photos and facts. US based.
Practical Action has good explanations plus simple ideas to make a difference
The Met Office pages include explainers and experiments to help kids understand weather and climate.
You can download a Climate4classrooms Activity pack with ideas and resources including lesson plans and data sets
Global Dimension brings together resources, case studies and background information from a range of organisations to help teachers and other educators bring a global dimension to their work.