Looking for an idea that’ll give her a little jolt!

I’ve been seeing a girl for quite a while, in a sort of “weekend marriage” kind of arrangement. I’ve taught her golf and played rounds with her, and we recently went out for monjayaki. I actually had never eaten monjayaki before (I’m not really into flour-based foods), but when I told her that, she insisted that I try it. It was pretty tasty, though. (笑) Last week, she messaged me saying a friend had invited her out for a meal on the 25th and asked if it was okay for her to go, so I replied, “Sure.” We’ve also had to reschedule a few times because of work. I don’t mean anything by it. I definitely feel that she’s comfortable seeing me and trusts me. Still, I’d like to make her a little jealous, so I thought I’d ask for some ideas. What kind of harmless prank could I pull that would make her say, “Huh?!”? P.S. Mac, I’m not very confident myself, so please keep the suggestions non-extreme. I’ve thought of buying as many roses as her age, though.