Inform them that after you give the signal to begin they will have 3 minutes to draw the object listed on the paper. Really emphasize that they have only 3 minutes and they will need to hurry. Then say, Ready, Set, Go! The guests will quickly rip into their envelopes and begin drawing. At the end of the 3 minutes, tell them to stop drawing and to write their name on the piece of paper and put it back it the envelope.
Gather up the envelopes, give them to the mother-to-be and announce that she will pick the winner by determining who opened the neatest envelope. Your guests will be really surprised that they are not competing based on their artistic ability, but on how neatly they opened their envelopes. For some laughs, pass the pictures around for everyone to see.