I think the ear hats are adorable. You could do one for yourself and your husband and then one that says "baby last name". Have the picture of the hats taken in front of the castle and use the caption "joining our cast in month / date"
I have done some maternity shots where the parents are kissing in the background (out of focus) and holding the ultrasound picture in the foreground (in focus). It's kind of cliche, but it would be so pretty with the castle in the background.
You could always do a few of the ideas in the thread and make a collage with one of the pic stitch apps.