Most cities have (free) walking tours where tour guides will happily show you their town and give a brief history as well.
Sometimes it’s nice to join an organized tour and let someone else do the planning.
If you want to see a new place from another perspective, rent a bike and see a new city on two wheels.
Perhaps one of the best things to do while travelling alone is to spend the day at the beach.
Whether it is hiking, skydiving, bungee jumping, or anything you can imagine! If you can dream it, go for it!