We took the tour last Tuesday (May 15) and my husband and I were the only ones for the 2:45 pm tour and no one was signed up for the 3:45 tour when we checked in. You don't need to sign up early. We did go out earlier and saw the 11:30 am "It all started with a Mouse" show which was a nice little animal show. But we had horrible timing with the train, so going out and back twice took a lot a time. (we always arrived after they closed the gates for the train, so we watched it depart and had to wait ~15 minutes for the next train)
We saw the nutrition center and the science center (which you can actually see from the windows). The tour was pretty simple - it was nice, but not exciting. We didn't regret doing it, but this was not our first visit either, so we were open to doing different things.