What would make a good exercise for someone like me, working for a company, with a team, and a roadmap to stick to, would be for the exercise to be small and simple.
I tend to work using the Pomodoro technique. Having exercises that can be tackled in 20-30 mins would be the best.
I’d be happy with exercises that highlight areas of improvement even if they don’t really act on them. @nick suggests to use linters like Rubocop, and later on to fix a single class of violations. That sounds great to me.
When working in a team this kind of exercises identifying areas of improvement could be great to share ownership and passion for improving the quality.