The first time traveling internationally by myself, I went to Seoul. It is safe to get around at night, it's beautiful in the fall, and it was relatively easy to get around. The Cheonggyecheon-ro is a stream that runs through the heart of Seoul. There is a well maintained and nicely paved walkway along the stream so locals can go for a stroll or a nice jog in the city.
It was a nice surprise to see a section of the wall covered in the tile art made by local kids. I would recommend going there in the day and coming back at night time to see the neon lights glisten off the water.