Visiting, what do I absolutely need to check out?

I already know the baseball, football, and basketball…. but aside from those; what are some must see places?

Of note… I'm a solo traveler. I try to find places where there's a lot of people or it's easy to chat up a conversation; but I also try to enjoy the main tourist attractions, as well as some favorite spots of the locals.

I generally youtube/tripadvisor everything, but sometimes reddit can give me a hidden gem…. or people telling me to leave the city I'm in…. but I digress.

TLDR: I'll take all the suggestions.