OMGG! I've been looking up so many science experiment thingo's...
i had a few options:
1. observe the effect of gravity on plant roots by growing it upside down
2. see human reactions to weird colour/taste combinations
3. see which kind of material insulates heat the best by putting your hand in ice water with different things over it e.g. rubber glove, cotton glove, cling wrap etc...

i havent decided yet. i can see flaws in each experiment, so im not sure...
im still looking.
there are soo many sites dedicated to science experiments for kids. gawsh...
the csiro website was pretty helpful, and gave me some other ideas, but theyre not really long term or appropriate.

maybe something with eggs?