If the router is not yours, you shouldn't mess with it. You shouldn't even connect to it, even though there is no security. Sniffing a AP that doesn't belong to you is considered an illegal activity and thus, it's discussion is not supported by this forums, according to the forum rules.
As for the pineapple device, I can't give my opinion as I never owned one (except that 100$ seems too much for something you can create using a laptop, a wireless interface and the aircrack suite + some other tools). But I can tell you that any router should be enough for you to set up your own test lab. I have two relatively old speedtouch routers and both work great for testing purposes, letting me learn in a safe environment. I can create my "victims", set up services and exploit them without worrying about messing something up.