Actually... All they have at the moment is that spinkler. The case is that "Best Friend" shows up all the time. If I try to make them enemies, it still doesn't show up - this time "My Roommate" or something like that. Well, anyways, I'll retry, thank you Zylphe!
Oh, okay. The Sim that I had play in the sprinkler is part of the household and is an enemy of my photographer. The sprinkler Sim isn't listed as a roommate of my photographer, though, so that's probably why I could get the photo to register properly.
If all you have on the lawn is a sprinkler, then try a party as BellaClo suggested. In my experience, it didn't really seem to matter if the Sims had Loves the Outdoors or not. If there's nothing else for them to do, the Sims generally play with the sprinkler.