The most obvious use of a piano in a preschool classroom is to play songs. Pick a group of songs you know your children know. Ask them to pick some during class one day. Take your list home and learn how to play the songs. Look up the music online or learn the melodies by ear. Bring the keyboard into the class and start playing the songs for the children. Let them sing along and dance. Change the instrument tone of your keyboard as you play. This will help introduce them to different instrument sounds. If you can't play the keyboard well, most keyboards come with sample songs that play automatically.
Let your kids take turns playing the piano. Explain the basic method of playing the piano. This includes playing with your left and right hands and hitting multiple keys to make chords. Make this very brief and simple. Give each kid a time limit of a few minutes. Monitor them while they play to make sure they aren't hitting the piano or keyboard too hard while they play. Encourage them to play softer if they hit too hard. Turn the volume down to about a quarter volume on an electronic keyboard to avoid making too much noise. Record the music and burn it to a CD for the kids to take home.
Many kids may be able to recognize tones even at a preschool level. Sit down at the piano and play a note. Sing it out loud and encourage the children to sing it out loud. Play a different tone and sing it in the same way. Ask them if it is the same note as the one you already played. Play a few more notes in the same way. Hit the first note you played again. See how many kids recognize it as the same note. Congratulate those who do and encourage those who did not to try again. Continue this activity, using the same first note, until everybody has recognized the tone at least once.
Use the piano to help teach the alphabet up to "G." Explain that the white keys on the keyboard are named after the first seven letters in the alphabet. Play an "A" key. Tell the kids that this is the key and sound of the "A" on the piano. Play the rest of the white keys up to the "G" key. Let the kids sing along with the ascending melody. Play from "A" to "G" several times and encourage them to dance and sing. Explain how this repeats on the keyboard. Start on different "A" keys and play up to the "G" each time. Let the kids play this simple melody on the keyboard to make the activity more interactive.