I've been thinking about some ideas and I've actually come up with a few that could be good...

• something to do with different colours - how do they make you feel??
• the waterproofness of different brands of "waterproof" make-up - lipsticks, mascara, eyeliner, etc.
• compare waterproof make-up to non-waterproof make-up - which lasts longer and is more effective?
• how long does sunscreen need to sit before it is absorbed into the skin? or how waterproof is it?
  
• comparing the effectiveness of different mould/bacteria cleaners
• do spray germ cleaners (like glen20) actually kill germs and how effective are they compared to wipes or liquid cleaners?
• does sitting in a dark room improve your smell, hearing and touch?
• do organic foods like fruits, milk and meats, stay preserved for less/longer than non-organic foods?

I think the make-up ones should be fairly straightforward to do, the sunscreen one should be harder because I would have to do it on skin, which means I might have to do it to lots of people because all our skins are different.
I'm not sure about the germ/bacteria ones because I would probably need a microscope. If I did it with mould though I might not need one because mould is easy to see.
I think the dark room one would be interesting but because it's to do with people I would need 30, which would be annoying.
The organic food is pretty simple as well, but it would be boring if there was no difference in the lengths of time it takes for them to go off.

So, I'm going to do some thinking and maybe some research and see what I come up with.