My support group (bereaved parents) makes ornaments out of foam sheets decorated with puff paint to adorn a tree each month. They have held up to all types of weather.
If you would like to see some examples, I have them on my son's website:
Original message:
> I need ideas for outdoor Christmas ornaments that third graders could
> make.Of course I have a limited budget. They will be outside for about two
> weeks.Any suggestions?