Activity table need clearing

[Andrew Hodkinson]

In my database I have a activity tables which logs various user activities, such as "last login". The problem is this table has become huge and I don't really need all the data which is kept in there.

Until now the table has been cleaned up by removing all activity older than 30 days, but I would like to change this to retain the last 100 results per user, and clear everything else. This means if a user hasn't logged in for 6 months, we still have a record of "their" latest activity.

The structure looks something like this:
activity_id int(10)
activity_user int(10)
activity_datetime datetime

I am a little stuck on the logical solution for this.