What is an adjective for cheerfulness?

Formal:

* Joyful

* Ebullient

* Exuberant

* Merry

* Blithe

* Vivacious

* Jovial

Informal:

* Cheery

* Happy

* Bright

* Sunny

* Upbeat

* Optimistic

The best adjective to use will depend on the specific context and the level of formality you desire.