In some fanfictions, the first kiss occurs during a holiday or special event. Maybe it's at a Christmas party at the FBI office. The festive atmosphere, the twinkling lights, and the general sense of celebration can create the perfect backdrop for their first kiss. It adds an extra layer of romance to the moment.
Often, it's in a place that holds special meaning for them. For instance, in the parking lot of the Jeffersonian. It's a place they pass through every day, and it can be a location where they finally let their emotions spill over into a kiss. Or it could be during a trip they take together for a case. Away from their usual environment, they feel more free to explore their feelings and end up kissing.
One common setting is at a crime scene. After solving a particularly difficult case, the tension and shared success might lead to a spontaneous kiss. Another setting could be in Bones' apartment. They might be having a quiet dinner or just talking about their day when the mood shifts and they kiss.