What is a good science experiment that goes for a while (so i can write i diary on it)?

[Cricket.Soccer.Guitar.Me]

there is no category limit, just science, this is my major assignment for years nine and ten, but i cant think of any ideas. the example i got given is: Does the combination of fruits in a bowl affect the rate of deterioration?
Can you give me some along those lines?
thanks alot
PS i know ive asked these kinda qs alot recently, but no one seems to be answering...