Deepening knowledge of the company's employees is the key to let them develop themselves and keep them in good mental condition. There are a couple of ways to provide them with it. Of course, an interesting and challenging project should be a major factor, as most of the developers' time is consumed there. Giving them an opportunity to take part in side projects, for example, via hackathons is also a great way to explore new areas.
Another great idea to expand their horizons is just to let your employees share knowledge between themselves. Here comes a fairly easy way to make it work, which is Lightning Talks!
In every company, no matter if it creates software, mechanical parts, chemicals, delivers law or economic ideas or any other similar example that comes to your mind, there is a need to ensure processes, which provide quality of designed solutions or products. These processes, in general, are called Acceptance Testing(AT).