Oracle creates nicely HTML-formatted workload reports (AWR) that list things like instance statistics, wait events, the top ten heavy SQL statements (by number of executions, number of disk reads, and so on).
Are there (easy) ways to customize this report?
For example, I want the top 25 SQL, instead of the top ten.
See the awr*.sql files in $ORACLE_HOME/rdbms/admin - they are the files that produce the reports. I've copied these files and tweaked them to alter the reports for my own use.
[Edit]
It occurred to me after my original answer that you might have only been interested in modifying the top-N parameter in this report - this is done via the DBMS_WORKLOAD_REPOSITORY.MODIFY_SNAPSHOT_SETTINGS package to modify the snapshot collection parameter.