It was either for my 4th child or my 5th child that I did this for. I bought 2 'pregnant body key chains'. It is clear, acrylic, or vinyl, or plastic bodice (no head arms or legs) and the 'tummy' is a clear rubber ball that can be removed from the bodice, and inside the ball is a pink/apricot plastic baby floating around in liquid. I think that I got them from Stupid.com for $3.00 to $5.00 each. Then I wraped them in a little box using light blue and light pink tissue paper. The only 'note' that I put inside was a blue on one side, pink on the other piece of cardstock (two pieces stuck together) with a question mark on each side (it was too early to know what we were having). Then I mailed them off making sure that my name was clear on the return address. It was a big hit and both my mom and mother-in-law carried them around in their purse telling people about it and showing them.
I had heard about one lady going out and getting a bunch of blue helium filled balloons and having them 'loose' in the living room so that when the kids came home from school, and the husband came home from work, then they could all guess that the ultrasound said that it was a boy.
This same idea could be done if you are telling a good size group of people... you could invite people over for a get-together, then decorate the place in light blue and light pink so that people can 'guess'.
I'm going to go read what others wrote. Good Luck.