Sap from the maple tree.
The item id for Maple Syrup in unturned is 1159. You can use this ID with the commands below to spawn the item, give it to a player or buy it on a modded economy server.
Useable: Consumeable
Size_X: 1
Size_Y: 1
Size_Z: 0.35
Water: 30
Energy: 25
Blueprints: 1
Blueprint_0_Type: Repair
Blueprint_0_Supply_0_ID: 392
Blueprint_0_Supply_0_Amount: 1
Blueprint_0_Build: 27
ConsumeAudioClip: Sounds/DrinkSwallow.mp3