No sumo in Osaka then, the one tournament held in Osaka is in March each year.
Osaka castle is nothing special, although when I visited the grounds had a lot of homeless (look for the blue tarps). I'd normally suggest Himeji Castle instead but I believe it is still under renovation; although open, there is scaffolding all over it.
Since it's your first time in Japan, the standard advice in this forum is to skip Osaka and stay in Kyoto instead.