How about observing behavior of liquid water under martian atmospheric conditions?
Experimental setup would require equipment found in a highschool science classroom
a large bell jar
a vacum pump, with vaccum guage
a small barometer, but it should be calibrated to measure VERY small pressures, a mercury barometer might work but oil would probably be better. You need to use a fluid with negligible vapor pressure at room temperature.
a small beaker (filled with water)
here is the idea.
Place the barometer, beaker of water, and thermometer in the bell jar.
Use the vaccum pump to SLOWLY create a vaccum in the bell jar.
As you measure the vaccum in the bell jar, try to bring the pressure down around 10 millibars. At this point the water is NOT boiling yet.
Continue to lower the pressure until the water begins to boil. This should happen somewhere between 5 and 10 millibars. Note the pressure and temperature when the water starts to boil.
Depending on the temperature (probably around 18 C) the water mail boil at different temperatures.
Combine this with a nice visual aids (graphes and posters) showing graphs of temperature variations, seasonally and daily. I think there is good temperature data available both for the pathfinder and viking landing sites. IF not you can get some orbital measurements or find other sources that show this variation.
A graph showing daytime temperatures, and maybe even local atmospheric pressure if you can find that, should show clearly that, at certain times of the day, liquid water CAN exist on mars. At some locations temperature can rise as high as 20C while at the same time having pressures above 7 millibars, water should be liquid at that point.