daypack       
noun [ C ] (also day pack) uk/ˈdeɪ.pæk/ us/ˈdeɪ.pæk/
 日用背包,小背包
a small backpack (= a bag used to carry things on your back), used especially when travelling or walking
  The return trip can be done in 10-12 hours if you're fit and just take a daypack. 如果你身体健康,又只带一个小背包,回程可以在10-12小时内完成。 
 A day pack should include sunscreen, sunglasses, a wide-brimmed hat and a bottle of water.  日用背包里应该放入防晒霜、太阳镜、宽边帽和一瓶水。 
 - More examples
 - Keep a photocopy of your passport in your daypack at all times for identification purposes.
 - The bag doubles as a wheeled suitcase and a backpack - a daypack detaches for use in the car or on the plane.
 - If you have balance problems, it might help to do more hiking with just a small daypack, because a big pack can make everything feel less stable.
 - A climber found a day pack of a woman who disappeared five or six years ago.