lol! I like the idea as well. Since you know the Zoo is open during the time you want to propose, and imo, I don't think it sounds cliche at all, it sounds to me that you wanted to bring back the first memories of you two being together, which is cute and girls love cute things like that. On top of that, even if the zoo was closed and you had to propose earlier, you two are down with a long engagement. Sounds like a win-win to me.