General synonyms:
* Cheerful
* Optimistic
* Positive
* Enthusiastic
* Joyful
* Bright
* Sprightly
* Lively
* Animated
* Buoyant
More formal synonyms:
* Ebullient
* Effervescent
* Exuberant
* Jovial
* Merry
Informal synonyms:
* Happy-go-lucky
* Perky
* Chirpy
* Cheery
Synonyms focused on attitude or energy:
* Energetic
* Vigorous
* Spirited
* Motivated
The best synonym for "upbeat" will depend on the specific context.