Sisterhood retreats with bonding and team building activities are good but I think most sisterhood is built in smaller groups as opposed to chapter wide mandatory activities.
Alumnae chapters have SIGs (Sigma Interest Groups) where women with similar interests can get together. You could form a couple SIGs for the chapter.
Book Club
Dancing
Dining
Tennis, soccer, other sports. (IM Teams are a great way for the girls to bond on the team. Also, the chapter can come out to cheer them on and it's a great way to get easy PR if the team name includes SK.
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Running/Walking
Gray's Anatomy, any favorite show really.
The SIGs can get together every week or two. Any of their activities would be open to all sisters but nobody would be obligated to attend. Mandatory bonding can be a turn off to some women.
As a chapter sisterhood can be built by going to the school's sporting events. Have an SK tailgate party.
Are there any camp grounds with in a reasonable drive? A camping trip in the mountains would be a great way to bond.
Encourage the women to hang out with each other even when doing things they wouldn't normally think to invite someone with them to do. Trips to the store (2AM trips to Ralphs were common when I was in school) or even just an afternoon of doing laundry can give a small group of girls time to learn more about each other.
Something you could try to implement that we did was we had a weekly prize for the sisters that shared the best activity during meeting. The girls would brag about what they did with their sisters and the winners would get a small prized. Candy, pens, small little things.
Making sisterhood a competition is a surprisingly effective motivator.
We also had a Violets & Vinegar box. During informal meetings women could put notes in the box that were then read out loud by the Pres or EVP. The notes could be anonymous and were usually "Suzy, you're the best" types messages.