She could do a simple project, which cavies like which toys better. So do the boys or girls like a certain toy better? I would try 4-5 same toys with both pairs of cavies and see who likes what better. (This is just an example)(for the toys cozie, hammock, etc)

So she could do the following things to complete her project

1) Form a question (exmaple- Which toy(s) do the boy and girl cavies like best?)

2) Form a hypothesis (I think the boys will like the hammock and cozie best, and the girls will like the tent and hidey house best.)

3) Write down the materials/requirements needed and the steps she will take. In other words design your investigation. (example, materials->toys, first I will try put a hammock in the boys and the girls cage. Watch for a couple of days and tally how many times each diff. sex cavy uses the hammock. etc.)

4) Present what she discovers (in this case who likes what toys best, example form graphs and write down how many times each diff. sex cavy used each toy. Example Boys used hammock 3 times, girls 5. So the girls like the hammock better than the boys.) Using a graph, table, or chart.

5) Write out a conclusion, was your hypthesis right or wrong?

You could do something similar but simpler like only use one pair of cavies the girls and determine which toys they like the most and which toys they liked the least.

Anyways I don't know if this helped at all. lol but it is just an idea.

Some other possible experiments-

Do the cavies like meadow hay or timothy hay more?

Just write a paper all about guinea pig care

Something about the different breeds.

Which veggy do they like best?