A huge focus of my work is talking to people about engaging employees and what that looks like. There is no one simple solution as to what engages employees best - we are all different and therefore motivated by different things. What engages and motivates one employee is not necessarily going to have the same motivational impact on someone else and part of the answer is therefore to talk more frequently to your employees about what makes them tick.
That being said, it comes as no surprise that recent research indicates that corporate sports event (such as a sponsored 10k run) has a positive impact on employee engagement and well-being. The research points to the fact that a shared experience, which is outside of normal day-to-day duties, increases rapport between colleagues and introduces a strong sense of purpose, as well as the positive side-effects of exercise! Often corporate sporting events have been seen as part of an organisation's corporate social responsibility policy but now it appears there is good reason to consider it as part of your engagement and well-being strategy. So perhaps planning that sponsored 10k run should be moved up the agenda?