Idea/foods to try for the science assignment:
Which food gives the highest voltage? (Examples to try: Coke (contains phosphoric acid), citrus fruits, soda water (Carbonic Acid) Haribo Tangfastics (citric acid) Vitamin C tablets (Ascorbic Acid) Sour Milk (Lactic Acid) )
Which combination of metals works best?
(In chemistry terms, metals that are far apart in the reactivity series will give better potential differences so, gold and zinc electrodes could give better voltages than zinc/copper as gold is far less reactive than copper)
Could try magnesium (pencil sharpeners) and copper, aluminium (more reactive than zinc),iron, gold (only if you're rich!
Which lasts longest?
Use different foods, measure voltage each day of the week/each hour etc...
Ultimately, guide towards which is best - consider other factors like how easy to carry around, some foods may rot (e.g. spuds) as part of decision as to which food makes the best battery.
As for measuring acidity (i.e. pH), look out for Universal Indicator - often part of garden soil testing kits (or one of the pH meters that comes with it). Litmus will only tell you if something is acidic or alkaline, universal indicator will tell you HOW acidic something is - e.g. strong acid = red, weak acid = orange.