Here are two invitation ideas you could use:
i. You can use a clip art of the famous Superman logo which of course is the big S on Superman's chest. You can make a note invitation card using the logo as the cover, then place the party details inside. In order to give it some dramatic effect, you can attach a red fabric to the back of the invite indicating a cape.
ii. If you can make invitation cards that look a lot like a Superman comic. The front of the invitation card can look like the cover of the comic, and the inside will have comic boxes that will be used to show the party details.
