General synonyms:
* Decorative
* Showy
* Beautiful
* Elegant
* Artistic
* Adornative
* Embellished
* Ornate
* Fancy
More specific synonyms:
* For plants: Landscape, horticultural, floristic
* For objects: Decorative, aesthetic, artistic, stylized
* For clothing: Fancy, elaborate, embellished, frilly
The best synonym for "ornamental" will depend on the specific meaning you want to convey.