Here are some options depending on the context:
If you want to describe a *feeling* of happiness:
* Smile: This is a physical manifestation of happiness.
* Laughter: Another physical expression of joy.
* Joy: This is a more abstract noun but it can be seen as a feeling.
If you want to describe *something that brings happiness*:
* Gift: A tangible item that can bring happiness.
* Success: A personal achievement that can make someone happy.
* Love: A powerful emotion that often brings happiness.
Ultimately, the best "concrete noun" for happy depends on what you are trying to express.