Drop in:
Please drop in for tea.
请顺便来喝杯茶.
Drop in at:
Last summer I went to Wuhan and dropped in at Yellow Crane Tower.
去年暑假我去了武汉,顺便去了黄鹤楼.
Drop in on：
Last summer I went to Wuhan and dropped in on my friends.
去年暑假我去了武汉,顺便拜访了我朋友.