Adding my .02
Dan is special to us. We have hakaras hatov. If we want to do something special for him and Mimi, it has to be something that they wouldn't have bought themselves ot something that they haven't bought yet, but wanted. Do you really think giving a gift certificate is meaningful and efficient? The amount of a VGC would be pennies to them; a gift certificate at a particular store will force them to go to that store versus going to any store they choose, whenever they want; you think they don't have magnatiles with three kids already?? I think someone should get in touch with someone who knows them and ask what is being said outside of ddf doors (Bubby Eleff, maybe?) She will know both families.
If we don't go that route, something personalized is always appreciated. Baby's name on a knitted or softie blanket. Little name bracelets for the two girl JJ babies. The third and Dan's? Bathrobes with their Hebrew names embroidered (in a toddler floor length size to last a while)