Well, there is the rare active super-fat girl. Conceded, you won't be able to go mountain climbing or take the roof tour at St. Peter's Basilica*, and they won't be exactly 600lb, but if you know where to look, you'll find enough, pretty, charming and active 350 to 450 pounders with whom you can do things, visit places, have good conversations and just generally be active.
*(Please don't even try: We did that once, and when the point came where she didn't fit and had to turn back, there was a 50m queue of people in a 50m corridor who had to turn around with us to let her out, because nobody fit past her in that corridor.)