Is there a college or university near you? They often have events, speakers, etc, open to the community. We had a "Moth Radio Hour" type event recently that was fabulous. Is there a community college, museum, garden, historical or arts society? They often have lectures or one-night classes, or are just fun to visit without kids to watch out for. Speaking of one-time classes, stores will do that, like cookware or hardware stores. Chefs will host classes, too, so interesting even for this non-cook. Take a tour! Some are oriented to a particular interest, like a food tour, haunted sites (lots of history to be heard), literary or movie sites. Some tours are "do-it-yourself", where the stories are printed/online as you go. We've seen Segway tours that look fun, mainly because you get to try a Segway. How about geocaching on your own, without explaining to the kids?
Look for daytime activities as well as well as evening. We'd rather do something and then eat, rather than the other way around, or have lunch then the activity. Seemed like getting a sitter and explaining and arranging supper and bedtime for everyone was more work than the date was worth
