How about:

This side up
No Step
This tube intentionally left blank
Do not use as a floatation device
This bike is equipped with LoJack anti theft locator device
Smile, you're on Candid Camera
Do not use heavy machinery while operating this bicycle
If you can't see this line... put the dam seat back on
Warning! Anti theft seat impalment device is activated
If you can read this... you're too frikken close

Sorry, it's late, my legs are stiff and I can't sleep. This is what I usually come up with under such conditions.