Is there a problem with maris ayin to enter a non-kosher restaurant and order just a drink?
Let’s say that in the first case the restaurant in question is a fast-food place and the second case is a “sit-down” place. The differences between these two cases would be:
In a fast-food restaurant, the cups are disposable and are not reused, whereas in a sit-down restaurant the cups are non-disposable and are reused after being washed.
In a fast-food restaurant, you don’t have to sit at a table; you could order your drink and promptly walk out afterwards, whereas in a sit-down restaurant you would have to get a table, order through a waiter, etc.
Would any of these points make a difference in the din?
The issue of maris ayin is that people will think that you are eating non kosher things in the restaurant, and it wouldn’t matter whether you are sitting or standing. If it is a normal thing to only order a drink in this restaurant, and people seeing you would readily understand that you came in only for a drink then it would be permitted.
Igros Moshe O:CH 2-40.