Gender reveal help!

Lynzee • Mommy to Ammarie, born on March 3,2016!
My husband and I will be going on a "baby moon" at  the end of October. I'll be about 18 weeks pregnant, so we're hoping to know the gender by then. 
We're going to his favorite place (and somewhere I've never been but want to go) Ruby Falls! 
We're staying at an older, classic looking hotel and doing a river boat dinner cruise. I'd like to do something so we can find out the gender together and privately while on vacation. I need ideas on how I can do this without me knowing before he does. I thought about having the ultrasound tech were the gender down and put it on an envelope and we open it while on the river boat cruise, but does anyone have anymore ideas?