If you are going into gautemala:
Don't wear any kind of american military or military look garb.
have everything ready for paying entrance and exit taxes.
I had no problems, but the area where we went had a military coup 2 days after we left. It was totally peaceful while we were there though.
Mostly though, don't look military they may arrest you for it, and of course with any CA country it is not best to try to attract attention.
Also there will be a lot of people at the borders who want to exchange your money, we didn't really figure it out, they are somehow looking to make a small profit, but their rates were normal. just kinda FYI.