Here's a mini-list of ways of staying in touch - don't forget to add on!
Airplane banner
Braille
BSL
Carrier pigeon
Chinese whisper
Code
Email
Flags (semaphore)
Letter
Makaton
Megaphone
Morse code
Postcard
Post-it
Shouting
Sign language
Signs
Talking
Telegram
Telepathy
Telephone
Text
Whispering