“Up and Atom!”
“Have a nice day.”
avada kedavra
Beware of falling objects
Say hello to my little friend
Nah.
E=mc^2, Motherfucker! 
A special gift for my special snowflake.
Morning has come.
“President Trump says hey.”
Keep the change.
“Smile and wait for flash”
US GOVERNMENT PROPERTY
If found, please return to…
I’d go with a Hello Kitty Hydro-drip.
Everyone loves Hello Kitty, right?
Polonium trigger warning.
Regards,
Shodan
Mine is bigger than yours! 
So long, and thanks for all the fish
“Do not drop.”
“Hello Amarillo*.”
- Amarillo is where they refurbish nuclear warheads, and besides the American ordnance people the people that are likely to see my message are the technicians.
“Shall we play a game?”
StG
Warning! This item may be hot!
Applause
Well ironic pacifist messages like that are OK;)
‘Peace Through Superior Firepower’ may also be appropriate.
Some really good ideas in here!
Light and heat
give life to most
but in this case
thou shalt be toast!
“Does not contain ice cream”
“Not an ice cream cone”
“No user-serviceable parts inside”
“Opening housing voids warranty”
“Equipped with tamper-evident seal”
"Gone fission’ "