There are calendars online that can be used as a lesson planning resource. Thinkfinity has a calendar they call a "teachers planning calendar" offering tid-bits of "this day in history" information and links. The Teacher's Corner provide links to goings-on and special events teacher might use in lesson planning that compliment the time of year. eThemes offers a calendar with kid friendly links that both teachers and students can investigate and explore. About.com has a Family Crafts calendar with links to special events that happen on each day of the month that could be a great inspiration for student centered lessons to engage students.
Teachers can keep students challenged and thinking by offering links from their classroom website where students can visit to stretch their brains with daily problem solving activities that require higher level thinking. These kinds of links could be used as "morning" or "seat" work, when students finish other projects, during indoor recess, before/after school, and more. See the list below for a few daily brain-teasers, puzzles, games and more. If you have a site to add to this list please leave a comment and I will add it.
A Daily Puzzle
A Game a Day
Discovery Education - Brain Boosters
The Set Daily Puzzle