This year my chapter came back a week early from winter break for Spirit Week/Recruitment prep. All of us stayed in the house (60 girls living in a house built for 20...oh my!), which was a bonding experience in itself!
If you're just doing a single day retreat that obviously wouldn't work, but it was such a good way to bond with everyone at once I just had to mention it!
Since it was recruitment prep, we talked a lot about our own recruitment experiences. I loved when we went around and shared each of our stories about recruitment, memories of sisterhood, what being a KD meant to us etc. It was amazing hearing how all of the different stories somehow had such similar aspects, plus it boosted our pride in our house.
In addition to the recruitment stuff we did throughout the week, we also had random sisterhood activities. One of my favorites was a confidence exercise we did midweek to boost morale. It's similar to some stuff that's already been mentioned, but might be a little easier to execute depending on your location. You wrote your name down on a piece of paper, then passed it around the circle. Sisters wrote down qualities they liked or admired about you, along with the occassional inspirational/sentimental message. (Our chapter used sheets of paper due to space limitations, but I've also seen it done using half-sheets of posterboard scattered on the floor so people can walk around to leave notes, or hung up on a wall throughout the retreat so people can write at their leisure). It was a really powerful exercise that definitely boosted our morale in the middle of the week and brought us closer as a chapter!
Hope that helps! Good luck!