I'm a lurker, but thought I'd share since you're looking for ideas.
Sometimes in the past while DH and I were driving around town, I'd point out the stick family decals on the backs of cars and tell him that we should order a set for us (me, dh, ds, and dog). So in preparation for #2, I actually went and made a stick family of us at familystickers.com, but for me I used a pregnant body and underneath it has our names, and mine says "Mama and Baby #2".
When we do get pregnant, I'll stick it on the van and tell him I ordered us a decal and for him to come see.
I considered not putting the names underneath, but knowing DH, he'd probably just think I gave myself a chubby body and not get the whole point of the thing.