Okay, after discussing it with a few people, I've decided choose from either
'What type of plastic wrap prevents evaporation best?'
How permanant are 'permanant' markers? What will dissolve the ink?


I narrowed it down to these two because these ones in particular because they would go over for a couple of days, maybe even a week or two and that is exactly what I want.

For the first one:
Independant variable: The number of days the plastic wrap stays over the container.
Dependant Variable: The type of plastic wrap.
Controlled variable: The length of the sheet of plastic and the amount of water/liquid.

For the second one:
Independant variable: The amount/types of liquids that goes on the sheet
Dependant variable: The different markers.
Controlled variable: The amount of writing on the paper.

I have a feeling I'll stick with the first one actually. The second one doesn't sound like it'd work out very well.