What do you want to use to construct the report?
What specific ''last activity'' do you want to report?
This SQL will show you the last time each user logged in, based on the information stored in the SESSIONS_HISTORY table in the Site Admin database. How far back in time that data goes is based on a configuration parameter in Site Administration.
select USER_NAME, MAX(START_TIME) from SESSIONS_HISTORY GROUP by USER_NAME
As written, this query will show the last login time only for users who have actually logged in. So, the output will not include users in you User table that have never logged in (or for whom there is no remaining history of a login).