A while ago I tested out LED throwies (previous post) and putting these inside a balloon and then inflating them. One of my tutors suggested putting lights inside the balloon for when it's used at nights and so that the balloon is easily spotted.
The photos aren't very clear, it was difficult to see the LED through the latex and past the brightness of the light. All that I used was two small watch batteries anda 3mm LED. I attached them together with black tape and placed it inside the balloon. The effects worked better in the dark and this is not the situation the balloon is being designed for but I will keep in mind this little experiment that I've tested incase I need to use it at a later date.