The whole magic of the pants and how they affect the girls' lives every time used, whether real or just their imagination, is pretty cool.
Tibby's letter to the girls near the end of Sisterhood Everlasting where she explains everything. The ending itself too, with the three remaining girls getting a home where Brian and his and Tibby's daughter Bailey have moved to. Also the idea that Bridget is going to have a child as well.
While Carmen is really being the unreasonable one here during the fight (and it's not awesome that she makes Tibby, already in grief about Bailey, cry), she does make a good point when she calls out Tibby on how cynical and judgmental she's always been towards the world in a brief "The Reason You Suck" Speech:
'"And why'' are you giving me this hypocritical lecture on treating people better when you go around saying "Screw the world!" because it's easier than feeling something?!"