How about the basic study of basic bacteria, get a house hold germ
(they're the ones with the apron on and a duster, aparently there is a difference between bacteria and house hold bacteria who knew??)
Show them the growth of the bacteria, get 4 or 5 dishes and implant certain conditions. Once the conditions have been imposed leave them to grow. They can them specualte why there are certain differences in certain conditions and how the bacteria have adapted to each condition. (Water, domestos, sand, heat etc place the bacteria in and watch, even try different food but the same bacteria and monitor growth and speculate)
It a simple one to set and to do but it can leave huge holes that can be filled with speculation and graphs and colourful stuff for them to do.