General Positive Expressions:
* Smile
* Grin
* Beam
* Laugh
* Chuckle
* Giggle
More Specific Positive Expressions:
* For pleasure: Delight, joy, rapture
* For approval: Approve, agree, concur
* For excitement: Thrill, exhilaration, elation
* For relief: Relax, breathe easy, sigh with relief
The best antonym will depend on the specific context of the word "grimace" in your sentence.