Plant Projects?

Our girl scout troop is about to work on a badge involving plants and I am clueless. I need something extremely simple because the idea is to send each girl home with her plant and have them track the growth every few days on a chart. Then she would bring her plant back to our next meeting in 2 weeks. We were going to place lima beans and wet paper towels in a plastic baggie taped to a window, but then I read that you can't use lima beans from a bag in the store because they have been treated with something to prevent growth. So, now I am stumped! Is there anything else we can use (potatoe, other beans, etc) that is inexpensive and easy to use in place of the lima bean? Any and all suggestions would be greatly appreciated!