• a network card you know works
• preferably a laptop, your own
• two straight twisted pair (CAT5) cables
• one crossed cable
• one coaxial cable thin ethernet
• two terminators
• one T-divider
• one cable tester
• one serial/console cable
• several M/F and 9/25 pin adapters
• one knife
• several small screwdrivers
• one cable connector kit
• one punch tool
• one RJ45 extension bit
• clear understanding of your networks:
  • physical structure
  • logical structure
• traceroute, ping, nmap and other software tools
• keys to the premises, even the places people never go to
• pen and paper
• mobile phone
• a friend back in the office

You did bring your laptop's power cord or adaptor, didn't you?