Give Ben some attention

Feeling kind of bad about what happened (even if he really didn't understand what went on) Ben sinks into his deckchair, lying face down against the plastic straps. Deciding that Emily's course was taking her away from the growing crowd by the pool, you decide to take Ben's mind off of things and stick with him for a while.

Swimsuits made for an easy job of stripping someone- trunks, not really any harder than a bikini when it came down to it. The gaps between the straps that make up the back of the chair are just wide enough to pull something through; something such as a swimsuit's drawstring, conveniently dangling down through one of the gaps already. It takes a little maneuvering to do without Ben noticing, but in a few seconds you've undone the knot of his drawstring and pulled it out and through the back of the chair. He hasn't moved enough to notice the garment's new looseness yet, but a subtle little tug starts it sagging, the top of his butt peaking out as you drag it down a bit in the back.

Deciding you don't want to let the drawstring go to waste, you magic it around his ankles, a knot on either end. The sun daze (combined with a little helpful impish magic) mean he doesn't feel a thing, and won't until he tries to move his legs too far apart- meaning if he were to run for any reason, he'd take a nasty fall as his trunks did the same.

The only question now is if there's anything else you'd like to throw at unsuspecting Ben before sitting back and watching the situation unfold. There's a crowd around sure, but maybe more people could be looking at Ben specifically? Or are there any other pool-goers who could share in his humiliation, to set up a domino effect of embarrassment?