Directly related to joy:
* Joyfulness
* Exuberance
* Bliss
* Elation
* Delight
* Happiness
* Merriment
* Cheer
* Contentment
* Pleasure
* Rapture
More general, but still related to joy:
* Happiness (while it's also a direct synonym, it can also encompass a broader sense of well-being)
* Contentment
* Fulfillment
* Peace
* Gratitude
The specific word you choose will depend on the nuance you want to convey.