First you must decide if you will learn at the docs office, then plan all the fun for your party guests. If you want to be a part of the fun, there are a few options to join in on the surprise.
1) At your doc visit, instead of sharing the news, have them write the sex of your little bun on a piece of paper and seal in an envelope.
3) Another fun idea that I recently read, is it have your bestie peak in on the big secret (but you have to promise not to probe her prior to the big party). Buy a huge box and decorate the outside with "Boy or Girl???". Have her fill the box with helium-filled pink or blue balloons. Hire your favorite photog and open the box at your party. It will make for great pics to capture those faces of true joy and surprise.
4) Take a tally from your guests when they enter the party. Maybe have a chalkboard with boy and girl headings and have your guests enter their tally. You could also have them write their name on the side of their best guest. Those that guess correctly can win a fun prize once revealed. You can take a picture of all the people that guessed correctly to add to your baby book.
5) My personal suggestion...if it's a girl, you can put RuffleButt Bloomers in small boxes and have your guests unwrap the boxes all together to reveal the news that you will be having a little "RuffleButt" of your own. This is a great excuse to have a rainbow of RuffleButts awaiting your little angel when she arrives. They also make for precious newborn photos!!
My biggest tip...record the reactions. This is definitely one to capture on photographs, a treasured memory moment.
Boy or girl, we send sincere CONGRATULATIONS!!