I do believe that on some nights after a certain time (i.e. after 10pm or similar) some of the bars/restaurants may have a cover charge to enter. I'm not a bar hopper, so really never tried to head to one of the restaurant/bars that late. But as long as you arrive before then, you shouldn't be charged the cover charge for any of the bars. If you are going that late, I think you can buy a "party pass" or if you have a Park ticket, you don't have to pay the cover charges.