General terms:
* Dermal: This is perhaps the closest synonym, as it directly refers to the skin.
* Cutaneous: Similar to dermal, this word also means "related to the skin."
* Skin: The most straightforward option, but it might be too simplistic depending on the context.
More specific terms:
* Epidermal: Refers to the outermost layer of skin.
* Dermatological: Related to the study or treatment of skin diseases.
* Integument: This is actually the scientific term for the skin and its appendages.
Figurative language:
* Protective: This describes the function of the integumentary system, so it might be used in a metaphorical context.
* Outermost: This emphasizes the external nature of the integumentary system.
Ultimately, the best alternative will depend on the specific situation.