Waddle It Be? Use a book to help you get inspired like this family used, Make Way for Ducklings. The bottle cups are pretty good too. Check it out at Layla Grace.
When you arrive at the party you choose a mustache or a set of lips depending on which you believe is correct. Just before the gender is revealed every one holds up their lips/mustache & a photographer takes a picture of the group's votes (including mom & dad!) Fun to look back on who was right & who was wrong!