Ask HN: How to make bench time useful in a software house?

I work at a small software house (~50 people). We're looking for a way to get people who are on the bench (i.e. without a commercial project) something meaningful to do. Our goals are: #1 to help them develop their tech skills, #2 to have the company benefit somehow so this time isn't wasted.

Our main constraint is that sales are highly volatile and people often get engaged in a commercial project before they can finish whatever they have started. Typically, and what is also happening in similar companies we are aware of, people begin an internal project, but a week or two later a new deal is won and most of the team is sucked into commercial work. The project never gets finished, new folk has trouble getting involved, etc.

For this reason we are looking for something that can bring value continuously on a time scale of days to months, but we are open to any ideas.

What would be a reasonable engagement / task / project to work on in this time?