General antonyms:
* Uplifting
* Inspiring
* Cheerful
* Encouraging
* Joyful
* Hopeful
* Optimistic
More specific antonyms:
* Exciting (if "depressing" means dull or boring)
* Reassuring (if "depressing" means causing anxiety or worry)
* Stimulating (if "depressing" means mentally draining)
* Energizing (if "depressing" means making you feel tired)
The best antonym will depend on the specific situation and the intended meaning of the word "depressing".