[eluser]TheFuzzy0ne[/eluser]
[offtopic]
The last time I checked, this was a forum for helping people. If newcomers get ranted at for asking for suggestions, then this forum isn't much help, is it?
I can't see anything in the OPs original post that asks for it to be done for him. Instead he asks for an "idea". I've recently been looking for help with creating a forum, and I would have been totally lost without the ideas of others (especially Dam1an), and I would have been seriously annoyed if my request for help was met with mild hostility.
My idea might not even be the way to go, it's just what makes sense to me. In any case, it's not one of those things that can be solved by beating the user half-to-death with the user guide, so the answer might not be as obvious to them as it is to you.
[/offtopic]
Back on topic again - it looks like the CodeIgniter session class updates the timestamp for the users last activity, so perhaps you could use the session class and extend the session class and get the information you're looking for?