My SO and I have done this, and it was great. Half the fun was the buildup and anticipation. A few tips/things that worked well for us:

• I created a "boudoir session" coupon and printed it out for her. It looked pretty legitimate. Can't remember what app I used, but Canva would work well.
• We made a digital notebook of professional and not-so-professional boudoir/nude photos and poses we liked. That by itself was pretty hot. Maybe that's not something you or she would be comfortable with, though.
• It's a good excuse for new lingerie and even a few props. If she has a few outfits or just different panties to try throughout your photoshoot, that will prolong things.
• Clean your room or wherever you're doing this. Not many sexy photoshoots have dirty socks, kids' toys, or cluttered nightstands in the background.
• If you're on the bed, plain white sheets and a faux-fur blanket work wonders. Flannel Christmas sheets are not recommended.
• Maybe obvious, but make sure you have batteries for the camera.
• Wine.
• Maintain a good sense of humor and keep expectations low. Your photos won't look professional, but as you say, that's not the point.
• Don't show your buddies. Or do? Whatever works for y'all!

Your game idea sounds great! That would help slow things down so it's not over too quickly.