How to keep track of user logins?

I have a client with a private Wordpress blog - that is to say, you need to log in to read it.
We are using Counterize to keep track of visits to the site and to find the most popular pages.
But now he wants to know which users are logging in, and perhaps how often they are logging in to read this blog.
I checked to see if there was any info in wp_usermeta ( like last-login-date ) but there doesn't seem to be.
Any suggestions how to keep track of user's activities?
Thanks