this blog. It's chock-full of fun, inventive things to do outdoors (and indoors) with young kids. Although it's meant to be about pre-schoolers, I find it utterly fascinating and thought-provoking even in reference to much older kiddos. I think we all love creative ideas for kids' activities. Even better, when it's laid out in such a lovely, visually-appealing way!
PS: Check out the blog list in her sidebar for more great resources!