General opposites:
* Cheerful
* Happy
* Joyful
* Optimistic
* Hopeful
* Bright
* Pleasant
* Lighthearted
* Merry
Opposites related to specific contexts:
* Forbidding: Welcoming, hospitable
* Bleak: Promising, hopeful
* Harsh: Gentle, kind
* Unpromising: Promising, encouraging
The best antonym will depend on the specific context in which you are using the word "grim."