I want to offer FREE balloons

You can set the balloon price to £0 and all the balloons can be added for free - there's no limit to the number of balloons that can be added.

However - you wont want the "Buy a Balloon" button so you can change it to "Get a Balloon".

As always remember to hit "Update these details" to save your changes.

As well as balloons you also setup a race for free. Explained here and free setup here.