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A 2-liter soda bottle is probably too small to really work well with worms.
10-gallon aquariums cost about $10 new (or a few bucks at yard sales), and are a practical size for a small demonstration bin. As long as you don't let too many castings accumulate, it will stay light enough to move around pretty easily (to bring to schools, etc.). Make a cover with a cardboard box or something to keep it dark inside and encourage the worms to be active right next to the glass.
5 gallon and 2.5 gallon aquariums are also commonly available, but actually cost a few dollars more than the more common 10 gallon. They're also probably too small for an easily sustainable bin.
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