If you are running 8i, consider creating an ON LOGON trigger to ALTER SESSION SET CREATE_STORED_OUTLINES = TRUE. You can then examine OUTLINES and OUTLINE_HINTS views to see not only which tables but also which indexes are in use. For a more detailed explanation of the method see the chapter on query outlines in Tom Kyte's book.

I was wondering what would be a good way to track activity against database tables - indexes too - over the span of, say a week. I'm trying to get a feel for overall database usage - where are the active areas of the database, what sets of tables appear to be more static. There are over 150 tables that are empty, and appear not to be used at all. The application was dumped in my clients lap, and never reached it's full potential.