Perhaps it was all the hoopla of the run up to the Birthday (anyone who has had a four year-old will know why that is capitalized. It was Such A Big Deal.) Perhaps it's just the pendulum swing of spring weather. Whatever it was, Finn came down with a cold and spent a day on the couch. He's feeling better today, though still not well enough to be out and about doing our normal activities.
He loved it, and ended up adding other features like a grass bird's nest and a sandy beach. I'm going to try to keep this set up next to our small, backyard pond until the mosquitos become an issue!