General brightening:
* Illuminate
* Light up
* Cheer up
* Liven up
* Enhance
* Enliven
* Gladden
* Beautify
* Lighten
* Embellish
Brighten someone's mood:
* Cheer
* Comfort
* Soothe
* Encourage
* Uplift
* Inspire
* Hearten
* Revive
* Rejuvenate
Brighten a place or situation:
* Illuminate
* Light up
* Decorate
* Adorn
* Enhance
* Improve
* Freshen
* Refresh
More specific synonyms:
* Glimmer (for a faint light)
* Glow (for a warm, soft light)
* Radiate (for emitting light or warmth)
* Sparkle (for a bright, twinkling light)
* Shine (for a bright, strong light)
* Beam (for a broad, bright smile)
The best synonym for "brighten" will depend on the context of the sentence.