If you use the administration module, i think you can add an after commit event on the account object, since the login functionality updates the account, so i guess it will called.
[Don't try this at home] A dirty trick to log activity might be: add an object to your domain model, from which any object inherits. Make a virtual attribute in this object, which, as a side effects, tracks user activity. Let the attribute return an empty string and display it somewhere in every form. [/Don't try this at home]
Think the easiest way is to set your homepage as a microflow which tracks the user and opens the homepage.