Maybe Fort Tilden is more crowded on weekends than it used to be, but Fire Island's Fair Harbor is a solid alternative. It'll take about two and a half hours to get there by LIRR and ferry, which is fine for a day trip, especially considering that it might take you that long to reach Fort Tilden anyway, depending on where you live. Fair Harbor will reward you with a truly beautiful, some might say pristine, non-commercial beach. There are a few amenities there (a public bathroom, a store, a restaurant) but not enough to ruin anything, and there are still no cars allowed. But let's face it, neither Fair Harbor nor Fort Tilden are likely to become "overrun" anytime soon, because most people are too lazy to trek out to either location. Have fun sleeping in on Saturday! That's just more beach for the rest of us.