Before doing any post office activities, have a lesson about money. Give each child a sheet with pictures of dollar bills and coins. Explain what each one is and how much it is worth and have children color and cut out the pictures. Partner children up and have them take turns being the post office worker and the customer. The customer wants to send a letter, and the post office worker has to tell him how much it will cost and take his money. Preschoolers will not be able to count out money and make change accurately; but if teachers guide them, it can be good practice to help them start understanding the process.
Set up a play post office in the classroom that children can use during open play times. Find a large cardboard box and cover it with paper. Cut out a slot in the front that is large enough to put letters through. Label the box "Mailbox" and set it up where children can reach it. Put out paper, envelopes and markers next to the box, and hang up a list of all the children's names and pictures. Kids can "write" letters or draw pictures for classmates and address and mail them. Station a child or teacher at the box to be the postal worker who charges each child for pretend stamps, which they can pay for with play money. At the end of the week, choose one or two mail carriers to take the letters out of the box and deliver them to each child.
Have the children write letters or draw pictures for soldiers serving overseas. The Any Soldier project lets you choose soldiers to contact. Choose one soldier for the whole class to contact or a different soldier for each child. Teachers will need to help children write out a message so it is legible. Buy stamps; and after each child has written his letter, have him come to you with pretend money in exchange for the stamp. Use a map to show where each soldier is, then talk about how the mail will get to the soldiers by trucks and airplanes.
Have your class team up with another class. One class can be the customers and the other class can be the post office. One class can visit the other, bringing mail, packages or other objects they want to pretend to mail. Provide boxes in the post office classroom so that children can wrap up their packages to prepare to send out. The children who are playing postal workers can pretend to weigh packages and charge the customers. Then practice sorting all the packages by size or destination.
If the school and all parents give their permission, take the class on a field trip to the local post office. The United States Postal Service's field trip program lets students watch workers sort mail and shows them the whole process a letter goes through from the time it comes into the office to when it leaves. If a field trip is not possible, check with the school's mail carrier to see if he can visit the classroom to talk to children about his job and show them his truck.