Cross Training - Buhurt
Buhurt (Steel Fighting or Medieval Fighting) is a brutal full contact sport where you dress as knights! Yes real armor and real weapons (blunted, there has to be some sense of safety)!
It is a great sport for Judoka because it gives a chance to test Judo skills outside of the “comfort” of the mats and Judogi.
Cross Training - Brazilian Jiu-Jitsu (BJJ)
Fundamentally, Judo is enough to develop a person as a fighter and as an athlete. That said, it is important to find ways to expand your abilities. Fighting in something you are not comfortable will teach you about your habits and weaknesses. So even as a Sensei in Judo, I would always encourage my students to find other sports and martial arts to train in. Just do not get injured! Nor neglect your Judo training. If you are going to cross train, be smart!