Throwies are awesome, they inspire folks to do more, about a year ago I tried my first flying Throwies, and had a few problems(indoors helium balloons filled to the max pop very loudly). I will quickly in this short Instructable show how to do it, as cheaply as possible, and avoid the problems I ran into.
Step 1: Ingredients.
1 helium grade party balloon, 18 inch or so
1 LED (I used a blinking red)
1 CR1025 3volt lithium battery, you could use a cr2012 or cr2032 but they are big and heavy and the balloon needs to be inflated to near bursting.
Some electrical tape(I love the 3m clear electrical tape, but I could not find my roll (perhaps its too clear! ;-))
Step 2: Assembly
Just like on a traditional throwie, the + side of the battery should touch the long leg of the LED and the short leg should match the -
Try it out, if it does not light, flip the battery, if still does not light try another LED and or another Battery
If it lights up, Trim the legs using some wire cutters, to just long enough to cross the width of the battery. Wrap it in a little electrical tape, wrapping up to and past the base of the led. (There is a mold mark around the base of the led, that is razor sharp, and when inside a balloon it can cut and pop the balloon.
Once you have it taped up, drop it into the balloon
Inflate the balloon, this size flyie will need a fair amount of helium but don't fill the balloon so far it wants to pop.
Tie off the balloon if you just want them on the ceiling you are done, Or you can put it on a string. to tie off or wear.
Warning the throwie might be a choking hazard, don't leave unattended with small children.
Step 3: Other Options, and Thoughts.
I used a flashing red, a bright blue or white or green would be much more noticeable/photogenic.
An LED out of a flickering candle would be neat too, the whole balloon would flickr like a hotair balloon. .
These will last a few days, (The led/battery will likely outlast the helium). feel free to pop the balloon and get the led/battery and take them apart for a future re-use.