Really cute announcement ideas here, even for second pregnancies: http://www.rashoodz.com/_blog/Rashoodz_News/post/Best_Bump_Annoucement_Ideas/.
Otherwise another idea would be a picture of your oldest holding an ultrasound picture of the new baby.
I only have a suggestion for if you find out that you are having a DD (I have had this in my head for a while, and I will do this if I ever find out I am having a DD - I only have DSs).
Take a picture of your DS holding up baby girl items and looking at them with a face that says, "what ARE these things?!" Items include ALOT of pink stuff (If you can). Along with it, a sign that says, "ITS A GIRL!"
I was 4 1/2 months when we found out the sex....but anywho.....Congratulations! Glad every is going well for your pregnancy. Please give an update on the sex (if you feel like sharing).