Bring a clear, quart sized jar into school. Each week, fill it with different objects such as marbles, paper clips, macaroni, dried beans, dried rice, pennies, ping pong balls, or anything else you can think of. Give the students the entire week to estimate how many of those objects are in the jar without counting. At the end of the week, reveal the actual number and ask the students to give their estimates and explain how they arrived at them. Afterward talk about which strategies for estimating were the most successful.
Bring a variety of fruits and vegetables that are all different sizes and weights to school. Arrange them on a table where students can see them. With the fruits and vegetables, include a set of labeled weights that the students can use for comparison. First ask the students to estimate how much each fruit or vegetable weighs just by looking. Then ask them to do the same by going to the table in groups to pick them up and compare them one another and to the labeled weights. Discuss how they arrived at their answers.
Choose several objects in the classroom such as a desk, a chair, a whiteboard, a telephone, a pencil, a book, or anything else you can think of. Divide the class into groups. Do not give them tape measures or rulers, but instead talk about the average lengths of things such as their feet, hands or thumbs so they can use those objects as a reference. Then ask them to estimate the length of each of the objects you selected.
Cover the classroom clock with a sheet of paper so that no one can see it, and ask all students to remove their watches if they have them. Tell them you will be asking them to estimate how much time has passed at various points during the next hour. Have them label a sheet of paper from one to five and place it on their desk, then ask them to do quiet reading or to work on any classwork they need to finish. At set periods of one minute, three minutes, 30 seconds, 10 minutes, and 10 seconds, ask the students to write their estimate of how much time they think has passed on the piece of paper.