Sure, it's harder to manage all the kids on your own. It would be even if it weren't a rainy day with everyone cooped up inside. But it's part of the job, and it's part of sharing the responsibility with Pearl. You know she'll do the same for you when it's your turn to go home.
So how are you going to get through this long rainy afternoon on your own? The kids clearly need something to do, or they're going to start throwing more than paper airplanes.
You could do something spirited, since your favorite activity in high school was the Pep Squad. You could draw on all of that experience to make it fun for the kids, and you'd probably look good in front of Ms. Martin, too.
But what if you didn't do an ordinary camp activity? What if you could get the campers to do something that actually helped with your other plans? You'd probably have to do some work to convince them, but if you did, then you could make the campers happy and get some real work done.
Or, you could just give them all popsicles and let them hang out around the cabin. They'd be happy because you gave them an extra snack, and you could go off to do your own thing. Of course, it's not really an activity, which means that it's not really what you're being paid to do. But maybe you don't care about that?