Deactivate the real bomb - the dining hall could be filled with tons of black balloons with secret messages inside. The kids pop them to find a specific message. I did something like this to start Nicholas's treasure hunt (balloons were canon balls, not bombs).
Click through for awesome Secret Agent or Spy Party Training Activities / Games Ideas for your kids Spy or Secret Agent Birthday Party (Face Identification | Secret Code Breaking | Bomb Detonation | Laser Beam Training | Target Practice)
8 Awesome DIY Secret Codes for Kids - Type your own Top Secret message into the template and it will automatically encrypt to match the Cipher! Great as Spy Party games or activities, for playdates or even in the school classroom as code breaking games $6.50 via SIMONEmadeit.com
Free printable LEGO emotions themed I Spy game. I Spy printables are great for providing visual sensory input to kids, making them a great choice for visual sensory seekers. They also help develop a child's visual tracking ability and improve visual discrimination. This printable requires kids to visually scan through the objects and find ones that are the same. Download this FREE resource at: http://www.andnextcomesl.com/2016/04/free-lego-emotions-themed-i-spy-printable-for-kids.html