I am in the same position with you, I want to do something "sweet", not just tell them with words, although this is our second. DH is sure his mom may not react as well as we hope (not in a bad way, in an overwhelmed panicky way) so he said "no cute stuff, just tell them, otherwise I don't want to be involved"...he has since said he will do whatever I want to do, but I can tell he'd rather just tell them straight.
I love the card idea, why does he think it will be a bad idea? I gave MIL (and later FIL) a wrapped gift, which was a picture frame with a picture of a stuffed monkey holding a positive digital test to announce DS. It went great!