General:
* Foliage
* Greenery
* Fronds (for ferns and palm trees)
* Blades (for grass)
* Vegetation
* Flora
Specific to trees:
* Branches
* Canopy
* Crown
Other:
* Pages (in a book)
* Papers (documents)
* Permission (to depart)
More informal:
* Greens
* Plant matter
The best synonym for "leaves" will depend on the specific situation.