gender reveal for army bf any ideas???

My bf just left for basics and won't be back until the second week of September. We were going to wait to find out the gender together. But since most of my family is in California and we are making a trip down there from Oregon the first week of September we are doing a baby shower. So I will know the sex before him. He wants me to surprise him with the gender when he comes back. Any idea on how to suprise him?