Maybe give an ornament to your grandma or whoever wrapped as a gift and have it be next years baby's first Christmas ornament. Open it in front of everyone and hang it on the tree !They usually have boy/girl ones with blue or pink ?

cake! when you cut it, out fall blue m&ms; or pink. Or a box filled with pink or blue streamers and balloons. and cut it. out falls whatever color.

Have your sonogramer enclose boy or girl in an envelope and pick a boy outfit and a girl outfit and have her open it for her eyes to see and get the cashier to wrap up the outfit in a box and come Christmas open it Infront of everyone

you could give an outfit or binkie or bottle in gender to relatives with a note that says something along the lines of 'from baby...for when I visit and my mom forgot to pack this...' or something cute like that :)