Elementary school students are widely reported to have been assigned to come up with new endings for well known proverbs. Sounds like a fun activity for the young ones of all ages. Here are a few of my favorites:
Don't bite the hand that ...looks dirty.
A penny saved is ...not much.
If at first you don't succeed...get new batteries.
An idle mind...is the best way to relax.
If you lie down with dogs...you'll stink in the morning.
There are none so blind as ...Stevie Wonder.
Better late than...pregnant.
You can't teach an old dog...math.