Well, most obvious is the AMC theatre that's near the Orpheum, in case there are any movies you've been itching to watch.

Or perhaps catch a show of your own at the Wilbur, Cutler, Opera House, etc. Other than that, I'm not sure what other options you have aside from the usual things that people do on an evening after museums are closed: dinner, bar (which you already nixed), enjoy the scenery/outdoors (which may not be doable due to weather), or entertainment (shows, movie, club).