WP Activity Log is easy to install, configure and understand.

WP Activity Log plugin is one of the best plugins I have used and, more important, the usability is the best I have used so far in terms of a logical process, I can say this because the site I used it on is managed by admin who know very little about WordPress, just the basics that apply to their own installation, and they manage it just fine.

Very very low maintenance when set and very logical process which is perfect for users new to WordPress systems.