Two projects my kids did;
1. Which is the best food for growing white worms?
Consisted of about 5 margarine pottles with potting mix and teaspoon of worms. Each container was fed a different food (cat meat, baby food, luncheon, porridge, bread).
2. Do plant fertilisers work?
Had a 3ft tank divided into 3 (glass dividers, no water flowing between). Each one had one stem of baby tears and one sprite (all similar size but also measured). Left section had plant tabs, right had liquid fert and centre was control. Measured once a week and results compared.
Might need longer time for the fertiliser one though :-?