The Truman Doctrine said that the United States would give aid to any country threatened by the spread of communism. There are at least three other things that Truman could have done.
First, he could have said that only countries in certain regions were covered by the doctrine instead of saying that it applied to all countries everywhere. Second, he could have simply opted not to give aid to any countries threatened by communism. Finally, it would have been possible for him to declare that the US would go beyond giving aid. He could have announced that the US military would participate in any and all civil wars against communist insurgents. All of these would have been possible alternatives to the Truman Doctrine.