Green Hour is a great website I have been visiting for the past year or so now. It is created by the National Wildlife Fun and it aims to get kids outside in the green stuff for an hour a day.
It is a US site, so a lot of the activities are seasonally out of sync for us here in the Southern Hemisphere. There is an extensive archive, so I just look through that for activity ideas.
Even when I don't end up doing any of the activities suggested, it's a great reminder to prioritise outside time for the kids. I think fostering a child's connection with nature is one of the most important things we can do for the future of the planet. If kids don't have that personal connection with and love for the wild places of our planet it is unlikely they will want to protect them in future years.
We do 'Green Hour' every afternoon when DS 7 gets home from school.