As it turns out, there's some pretty great ways to freak the hell out of your boyfriend. There's a couple of things you have to get into place first, though, before you can execute them.
First, it helps if you live together. At the very least, you should be sleeping together on a regular basis.
Second, it helps if the time is somewhere in the nether regions of the day, say, 3:00 or 5:30 a.m. The ungodlier the better.
What you do, is you, the lady, fall asleep around midnight, like a reasonable person ought to. Then, when your beau comes to bed at the ungodly hour of his choice, you roll over and say to him, all groggy-like, "I'm gonna pee."
The longer the better.
Your boyfriend will, undoubtedly, be distressed as the minutes pass by. He will wonder if he should wake you and remind you of the task at hand, if he should check for a wet spot or if, perhaps, he should just let you be.
Later, when he's drifting off while trying to make the right choice, steal clumsily away from the bed. Be sure to bump into some walls on the way to the bathroom and make a general ruckus.
Then, finally, pee.