Here's a little script I run when I need to drop a dev database:
SET NEWPAGE 0
SET SPACE 0
SET LINESIZE 80
SET PAGESIZE 0
SET ECHO OFF
SET FEEDBACK OFF
SET HEADING OFF
SET MARKUP HTML OFF
SET ESCAPE
SPOOL DELETEME.SQL
select 'drop table ', table_name, 'cascade constraints \;' from user_tables;
SPOOL OFF
@DELETEME
SPOOL DELETEME_SEQ.SQL
select 'drop sequence ', sequence_name, '\;' from user_sequences;
SPOOL OFF
@DELETEME_SEQ
commit;