Mount Popa is the classic day trip from Bagan: a solitary volcanic outcrop rising above the dry plain, with Taung Kalat monastery and nat shrines stacked on its summit. The place is sacred in Burmese spirit worship, with shrines to the official pantheon of nats, and the climb winds up a long covered stairway shared with pilgrims, vendors, and opportunistic macaques.
The reward is partly the view back across central Myanmar and partly the abrupt change of atmosphere from Bagan's archaeological plain. It is touristy, untidy, and still compelling. Go early if you want cooler air and fewer crowds, and expect to remove shoes for the climb.