You mentioned not being interested in zoos, otherwise, Tobu zoo could be worth a visit since they have a firefly exhibit which is something I don't think I've seen anywhere else. One of my friends likes the Kadokawa Culture Museum which could be a part of a day trip to other parts of Saitama like Kawagoe and the Railway Museum but I haven't personally visited these yet.
I don't think Fuji Q is really suitable for a young child.
Outside of that, I can't really think of good day trips unless you're willing to spend money on a bullet train or flights. In that case, you can plan a day trip to any city that's closer than Okinawa. Nagoya would probably be the easiest day trip by bullet train. There's
Legoland, a great aquarium and a maglev museum. You can even fly one way to visit the "airplane theme park" in terminal 2 that has a great kid's play area and exhibits including the first Boeing 787.