Complex sentence: They end up eating ice cream at Serendipity 3, they then exchange goodbyes, then realizing that they both left something at the ice cream bar, they went back only to find each other again.
Split into simpler sentences: They end up eating ice cream at Serendipity 3, and soon exchange goodbyes. However, both realize that they have left something at the ice cream bar, and return only to find each other again.