My wife has a sundowning component with her MCI. She just tends to get frantically busy and wants everyone around her to join in. I try to focus her energy on small household chores: hanging up or taking down clothes, putting up dishes (which may end up in interesting places), things she used to do every day. Also just giving her a little attention seems to help and very often a short drive in the country and petting a neighbor's horse will quiet her.
Timing of meds to set in early evening helps.