You can have the ultrasound tech not tell you and just have them put the revealing ultrasound saying "it's a boy" or "it's a girl" in an envelope and open it with him.
You could go buy an outfit or shoes boy or girl wrap and have him open & have ultrasound pictures inside.
I would say take him some lunch and a cupcake or.something with blue/pink on it so when he bites in itll reveal the gender(:
You could put the older kids in all blue shirts or all pink shirts on a random day before he comes home from work and see if he catches on! Wait a few days after you find out so he's not expecting it.
You could do a cake. When your fiancee cuts a piece, if its a boy make the inside filling blue. If its a girl then make it pink.
You could do a gender reveal type party and find out together.