There’s some science here (air pressure and propulsion) but it’s fun!
You will need a balloon, string, a straw (preferably a fat one!), sellotape, a peg and ribbons and paper to decorate
Cut a 3cm length of straw and around a metre of string
Thread the straw onto the string
Blow up you balloon and hold it closed with the peg
Decorate the balloon to look like a bird
Stick the straw (the bit you threaded onto the string) onto the top of your balloon. It works best if you stick it to the back third of the balloon
Tied one end of the string to a chair (or similar!) and hold the other end tightly
Pull the balloon bird to one end
Gently let the peg loose and the balloon bird will fly along!