You'll need a leaf of lettuce and a balance. The precision of an electronic balance is nice for this particular activity, if you have one available.
If you have a balance that can remain dedicated to this activity, you can place the lettuce leaf right on it. Record the mass. Each day when the students come to class, they should record the mass of the lettuce. Continue recording the mass every day for a month.
[If you cannot dedicate a balance to the activity, you'll need to first find the mass of a weighing paper. Record that, then place the lettuce on the weighing paper and record that mass. Lift the paper with the lettuce on top and keep in a safe place while the balance is being used elsewhere. Return the paper and lettuce to the balance each day to find the mass. You'll have to subtract the mass of the weighing paper from each measurement to get the mass of the lettuce.]
Once you've collected all the data, you can graph it if you wish. Is the water lost at the same rate throughout the month or does it change?
You can also determine how much of lettuce (by mass) is water.
Mass of lettuce at start - Mass of lettuce at end = Mass of water
(Mass of water / Mass of lettuce at start) * 100 = % of lettuce mass that was water
If you've caught your students' attention with this one, you can proceed to follow the same procedure to find the water content in other items. Maybe your students will want to compare the water content in different types of lettuces or different types of leaves or different types of fruits or vegetables. Lots of possibilities - you could have something going every month of the school year!