This is what I made to put some little goodies in for my fiance for Valentines Day :D I have no idea where I learnt to do this as I was really young and can't remember XD
Of course you can do it for anything, I plan on making one look like an easter egg for him in easter :D
Now this is the tricky part. The bit you cut out in step 5 needs be as thick as the rest of the balloon as the idea is that you stick that bit of paper on with one a few strips of newspaper so when you pull the thread/ribbon it gives way and the "lid" comes off. Hope that makes sense XD Once you've glued it on, paint it.