Some great comments and suggestions here. I am often involved with such things since I Coach Executives and their direct reports. Anonymity is King at the outset. How employees experience almost every aspect of how you set the stage and then conduct this will determine how well it works. Simple is better. Three samples might be...

What is most getting in your way this week?

Who deserves an Award for what they did this week?

What have you been measuring this week?

And by all means somehow publish all the responses you get from whatever method you use