I'm guessing that you've already read the first one, The Sisterhood of the Traveling Pants (but if you haven't, I would really recommend going back to it).
Here are a few other titles that I think you may like:
Girls in Pants by Ann Brashares.
In the third book in the series, the girls from The Sisterhood of the Traveling Pants graduate from high school and spend their last summer before college learning about life and themselves. Also, if you'd like to continue with the series, the 4th one is coming out in January, it will be called Forever in Blue.