Creatine certainly does not turn to poison...it turns into creatinine, a natural metabolite of creatine. You've got plenty of it in your system...it just doesn't do anything for you. If it was poison, all of those companies making liquid creatine syrums would be getting sued for poisoning people.
The only danger with letting protein drinks sit around would be bacterial growth. Whey protein is just highly processed milk. It should be fine if you keep it sealed up in the refrigerator.
That said, if you don't have 5 minutes to mix up a protein shake, how do you have time to work out?