Where are some good places in the US for day trips?

My mother and I enjoy going to places for the day. We usually fly in early and take the latest flight out that day. We haven't been anywhere recently mostly because we don't know where to go. We've been to New York City several times, Seattle, Atlanta, Boston and Minneapolis. I'm sure I'm missing a few. But I'm just looking for some cities to go visit for the day so we don't have to find a hotel. We don't mind a lot of walking or how warm it is or anything. Just kinda looking for some new adventures.