We have had success growing beans and sunflowers with a quick germination time and very rapid growth (so germination by the next week and very visible changes thereafter).  However, we also have a sunny window ledge that we can put them on and people who can come in during the week to water them.  It still is a multiple-week project.  Interestingly enough, one of the students was so impressed with the bean plant growth in Sunday school that he decided to plant some on his own and sell the seedlings.  He convinced his father, a farmer, to give him some space in the greenhouse and earned a decent amount of money with his enterprise.  It wasn't exactly the message we were trying to convey but at least we know he was paying attention!