The riddle Calibos assigns to Andromeda isn't arbitrary. The answer isn't just "a ring", it's specifically his ring. He'd wanted to marry her, and while the ancient Greeks didn't use them, educated Joppans could have heard of wedding rings from Egyptian merchants.
One might wonder why, when Calibos sneaked into Perseus's camp, he didn't just destroy Medusa's head instead of jabbing it to create the giant scorpions and then sticking around to attack with his whip. Without the head, Perseus would've been powerless to fight the Kraken. But getting rid of Perseus was just part of his plan: had he succeeded in killing them all, he may have intended to bring the head to Joppa himself, use it exactly the same way Perseus intended, and then demand Andromeda marry him as his due reward. Thetis may have even been able to press Zeus to return him to human form for saving the princess and the city. It's even possible that Thetis had something of the sort in mind all along, when she imposed a thirty-day deadline instead of insisting Andromeda be sacrificed immediately: she was buying her son time to acquire the one weapon to which the Kraken was vulnerable.
That's an intriguing theory, but I don't think it holds water. Perseus was blessed with an indestructible sword, a flying horse, a Hephaestus-forged combat owl and the the Favor of Zeus and he still almost lost. I doubt Calibos, who constantly attacked Perseus from behind and was made to beg for his life like a bitch, would have the balls to face down the Kraken, especially on foot... Unless his overbearing goddess-mother pushed him into it.
It may have also been Calibos' attempt to not just sabotage Perseus, but also doom Joppa. A sort of "If I can't marry her, you won't either" plan and a "I'll drag this city down for daring to side with him" plan combined. Given that Thetis was willing to still let the Kraken be released even though Joppa is her favored city, she may had been willing to let her favor go if it meant helping her son's revenge.
Many viewers might have been perplexed (whilst being impressed) that Athena had the nerve to stand up to Zeus, her father and king of Olympus, when he commanded that she hand over her beloved pet owl Bubo to Perseus. Readers of classical mythology will know that Athena was one of Zeus' favorite children; she was literally birthed from Zeus' skull and more importantly, is his daughter from his first wife Metis, whom in classical myth he had swallowed whole while she was pregnant in an attempt to stop a prophecy around their possible second child overthrowing him. With that, she held a special place in his heart, and could get away with a lot more defiance than the other gods.
During the 3 witches scene, one of the hags mentions Perseus' "Red Cloak". Up to this moment in the film, Perseus is not seen with a red cloak. Indeed, it is quite a bit later before we see the red cloak.
