He's popped the question and you want to celebrate with your family and friends - making the announcement official.  A fun way would be to have a "roast" of the couple, surrounding the head table and room with large and embarrassing photos of the couple at various stages of their growth.  

All keeping in good taste and so as not to cause long-lasting resentment, ask each of the guests ahead of time if they would like to speak about the loving couple. They could talk about some special or funny moment they remember about the bride or groom as they grew up, or how they feel about their friend getting married and walking down the aisle. 

Invitations could be of the couple's faces cut out and pasted to wedding dress and suit. Place faux diamond key rings  and other diamond-themed favors at each place setting, and have a faux wedding cake (cardboard layers wrapped and decorated) with a comical topper .

This evening should keep everyone in stitches, and will be remembered long after the wedding ceremony has taken place.