Coconut Timber Skin

You touch a willing creature. Until the spell ends, the target's skin has a rough, coconut-like appearance, and the target's AC can't be less than 18, regardless of what kind of armor it is wearing. The bonus is increased by 1 for every higher spell level it's cast. 

The user can also cast it as a cantrip if they are a level 20 Druid of the coast, but they require the material components (coconut).

Cheech's Secret Palm Technique

"One with the coconut" - Sand Toe Cheech

This technique can be cast at spell level 7 and up, You hold a coconut in one hand. Until the spell ends or you let go of the coconut, your skin and hair have a rough and leafy, palm tree like appearance, and your AC is doubled.