Loop through pre defined values

You could do this, though probably not as slick as you'd like:

declare
  type nt_type is table of number;
  nt nt_type := nt_type (1, 3, 5);
begin
  for i in 1..nt.count loop
    dbms_output.put_line(nt(i));
  end loop;
end;

If you create a type in the database:

create type number_table is table of number;

then you can do this:

begin
  for r in (select column_value as var from table (number_table (1, 3, 5))) loop
    dbms_output.put_line(r.var);
  end loop;
end;

Also, as A.B.Cade has commented below there are database types that come with Oracle that you can use, such as sys.dbms_debug_vc2coll:

begin
  for r in (select column_value as var from table (dbms_debug_vc2coll (1, 3, 5))) loop
    dbms_output.put_line(r.var);
  end loop;
end;

This comes from A.B.Cade's comment on the currently accepted answer, but I find it a lot cleaner and deserving of more attention:

BEGIN
  FOR i IN (SELECT column_value FROM table(sys.dbms_debug_vc2coll(1, 3, 5))) LOOP
    dbms_output.put_line(i.column_value);
  END LOOP;
END;