To make these secret message balloons, all you need are balloons and a permanent marker.
(NOTE: You can use any color balloon you want, but I discovered that it is much better to use one of those dark purple or dark green balloons that look black when deflated. ...You'll see why in a minute.)
Next blow up your balloon, but DON'T TIE IT OFF... Hold the end shut with your fingers and write a secret message!
Then, let go of the end and let the balloon deflate. The receiver will have to blow the balloon up to discover the secret message!
(NOTE: If you look closely at the balloon below you can see the writing... However, it's not very noticeable because, when deflated, this dark purple balloon looks black just like the writing. That's why I like to use the dark purple or dark green balloons. I tried writing on other colored balloons, and you can always see the writing VERY well when deflated... And if you can already see the message, then it's not a surprise!)
...AND TO FINISH IT OFF...Make as many as you want! But before throwing your balloons into the package, tie them together with ribbon to make a balloon bouquet. Also be sure to make a cute tag that says something along the lines of "Blow these balloons up to reveal the secret messages!" (NOTE: As an easier alternative, use a plastic baggie instead of making a bouquet and tag. Just use your permanent marker to write the instructions on the front of the baggie and then throw the balloons inside. ...Viola! Easy as pie and done in seconds!)
Have fun with it and be creative! :)