We do lots of sisterhood activities throughout the year. One that always makes sisters feel loved and special is one called the web of sisterhood. Get a big thing of yarn from Joann's and have your sisters sit in a tight circle. You start, and say something you like about a sister who you aren't best friends with. You throw the ball of yarn across the circle to her, and then she says something about another sister, and so on and so forth until everyone's name has been called. You can't repeat anyone either, so no one gets called twice and no one gets left out. The whole point of the exercise is to let each sister know that they are special, appreciated, and loved by everyone and not just the people they are close with in chapter. So that can help break down the clique barriers a little bit.
Here's another good one. Have the girls stand in a circle with their backs to the center and their eyes closed. Depending on how many girls you have, pull a few out. If you have 50 sisters, maybe pull out 10 at a time and put them in the center of the circle with their eyes open. The moderator of the activity (probably you) will say to the girls in the center of the circle nice statements like, "tap someone on the shoulder who makes you laugh," or "tap someone on the shoulder who you can count on." The girls in the inside of the circle can tap as many girls per statement as they want. The point is that the girls in the outer circle don't know who is in the inner circle and don't know who is tapping them on the shoulder. Every couple of questions, switch out so that everyone gets a chance to be on the inside. By the end of the activity, everyone should feel appreciated and like they are an important part of chapter.
In the past, we have filled up a box with the names of every sister in chapter and had a girl pick out a name at random. Then we wrote them a letter about all their good qualities, all the great things they did for chapter, and why we love them so much. And if there was anything we needed to apologize for or to clear the air we could do that in the letter too. Then you sign your name. This might be good for your chapter too, since you are concerned about clique problems.
Anyways, these are just a few ideas. I hope I helped! Don't get too caught up in the location. The activities are the most important part of a retreat. Oh, and food. Hangry girls are not going to bond as well as girls with full tummies. My chapter learned that the hard way one year. I really hope that you can work it out!