A friend & I visited 4 archeological sites in the Yucatan, 1 cave, and 1 cenote, all in the Yucatan and all by bike. We were able to either bring our bikes inside the gate or lock them right by the ticket kiosk. I've done the same all over the world when visiting various places. Maybe it's dumb luck but nothing has ever been stolen in those circumstances. I recall that when visiting Pompeii in Italy, they wouldn't let us bring our 4 bikes inside the gate. We were directed to a nearby ice cream vendor who told us that her 2 children would watch our stuff for a price, of course. I negotiated a price, a fairly modest sum. The children weren't happy because the price agreed to by their mother was for all 4 bikes, whereas the kids wanted it to be per bike.