Gather a collection of child-sized instruments such as guitars, drums or triangles so there is one available for each student. Hand the instruments out and have a session of free playtime in which the children can simply have fun trying out the instruments. Pots and pans can of course be used, as well. After a few minutes, announce to the preschoolers that it is time for everyone to switch instruments. Choose one direction in which the students should pass the instruments to avoid chaos.
Alternatively, gather enough of one type of instrument, one for each student, and instruct the class how to play it. Choose an instrument you know how to play or ask the parents to showcase their talents, if possible. This can become a parent-child event that can be held after school hours during the week. This could also be a weekend playgroup if your students' parents are unavailable on weeknights.
Gather a group of musically-inclined volunteers, such as students from neighboring schools or parents and caretakers and host a live concert in your school. It is important that the musical group engages the children and gives attention to each child equally. If the group can provide props or instruments for the children to play with this will be more stimulating and educational. Invite parents to join in as well.
If finding available talent is proving impossible, a recorded concert is perfectly sufficient. You can purchase a DVD or even stream an event from your computer. Additionally, you can provide your students with an instrument to play along with the performance or teach the preschoolers the words to one of the performed songs. This way, they can sing-along with the recording. Again, parents can be invited to participate as well.