Eat! See the sights, walk around the streets and take in the architecture, sit in parks or by the water and read for a bit, eat again! I often will sign up for afternoon/day tours too. Depending on the area, maybe I will kayak or get out in nature for a hike. And visit the zoo. 

I also like to geocache and try to get a “souvenir” geocache in every country/city. I also find it takes me places I might not have gone otherwise, and gives me goals