Iwould advise you to wear good shoes when visiting Yaxchilan ruins. You'll need a good grip. The place can be very muddy and slippery. To get to some parts you'll nearly have to climb and those tree roots can get dangerous too.
And If possible, cover yourself as much as possible. Jungle is home to many love-to-bite insects. I was like red-dotted dalmatian an the end of the day.
If you spend a night in Frontera Corozal or get ther early in the morning you'll probably hear a very loud noise that comes from the jungle. That is just howler monkey screaming. I really didin't imagine that they sound like this.
So, if you are sleeping in a hammock (like I did), don't fall out (I didn't :-P)