The microwave isn't that high a load. Even a kettle would draw about twice as much as a typical microwave. I'd go for the oven, a heater or the laundry dryer. (or the whole lot)
"Hi, I'm doing a load test on your house and need you to turn on your vibrator madam."
"I beg your pardon?"
"A load test madam. You'll need to get your vibrator and use it so I can ascertain if there's a power issue."
"Oh... Which one?"
"All of them madam. At full power."