Unlike so many stories that retroactively put people and items from the Superman series into the Superboy one, Adventure 283 (April 1961) really is the first appearance of the Phantom Zone, which would quickly become a major element throughout the Superman books.
A box of deadly Kryptonian weapons lands on Earth, just outside Smallville (where else?), and Superboy discovers that it was Jor-El (who else?) who sealed them away.
Superboy examines the weapons, including a disintegrator gun and an enlarging ray, but its the Phantom Zone projector that is the important one.
We learn, in flashback, of Dr Xadu, the first person sentenced to the Zone for failed experiments on suspended animation, and of General Zod, who attempted to overthrow the government of Krypton with a Bizarro army. This is the first appearance for both characters.
Superboy then accidentally sends himself into the Phantom Zone, so we get to see how they exist as insubstantial beings who can see and hear what is going on in the real world, but cannot be perceived themselves.
Superboy manages to escape by mentally causing a typewriter to work, detailing his situation, and Pa Kent uses the projector to free him. Never again would anyone in the Zone find it so easy to affect real objects.
While Dr Xadu would appear rarely, General Zod would become one of Superman’s greatest foes.
Congorilla’s final story gives him a decent villain, so it’s sort of a shame that it ended here.
A greedy scientist finds King Solomon’s Mines, and wants to keep the treasure for himself. He devises a way, much like the magic ring, to transfer his brain into a lion’s body. As the lion he scares off anyone who approaches.
Congo Bill at first believes he is facing a nasty lion, but eventually realizes there is a human mind behind it all.
At the end, he raises the possibility for the lion to return, and if the series had not ended at this point, I expect he would have. It’s not like Congorilla had much of rogue’s gallery.
Congo Bill and Congorilla would both return a few months later in a story in Action Comics, and continue to appear on occasion throughout the DC Universe. Janu would have to wait until the 1980s, and Swamp Thing Annual 3, for his return, as an adult.