We actually just held our Monday-Friday Healthy Hack Week last week (http://healthyhackathon.khanacademy.org). One of my favorite weeks ever, particularly because we encourage "hacks" of all kinds that don't have to include coding, and the entire company dives in.
They've been internal only so far. Maybe some day we'll open it up.
After a week of hackathon'ing intensely I'll walk in tomorrow more energized than ever (and eager to figure out how to use some of the team's work in upcoming projects). The commitment to staying healthy while hacking feels like it's paying off for us.
Plus it demonstrates our commitment to long-term investment in teammates.