IITFT – In my personal definition, nothing. You are all more than welcome to have your own, but I personally believe that fits the term “MO” in the best fashion. Like LUL says, my definition is “unique”, and I don’t expect others to agree. And yes, your grandfather may be MO (per my definition) if he considered Rav Yoshe Ber his Rebbe. It seems you don’t believe that is the case.
LUL – Regarding Tznius: Covering hair at least partially is a Limud from a Pasuk, but not a Lav or even a Bittul Asei. I’m not sure about mixed swimming (with a Burkini to avoid the other issues) being a real issue at all. Knees, upper arm and elbow according to many would only be required/enforceable if minhag hamakom (which is a totally different discussion!!). Shorts, if Derech Pritzus, would be a real problem, but you would have to show me that is a real issue in your community. B”H I don’t see them on religious women (even those who go to Young Israel, which may be your definition of MO) at all.