There are tons of GPS and hiking apps available, but the one free app that I've found really helpful when I want to take a last minute day hike is AllTrails.
AllTrails allows you to go to any location on the map (there are a lot of world-wide destinations added as well!) and view the maps, trail information, reviews as well as information about the difficulty of the trail and rules such as whether or not dogs and horses are allowed on the trail.
I imagine that if you're on business trip or just of town for a few days, you can use this app's "current location" find method to locate a local trail, get directions and head right to it!
I took some screenshots of the app to show why I like it. There are hundreds of trails near my hometown, and often, choosing one is frustrating just because there are so many. The first screenshot shows all the trail locations with a dot. Next time I'm in Boulder I'm gonna use this to check them out!
The second screenshot shows that if you click one of the dots, it will give you the trail name and zoom in a bit. Then, you can click to view the trail's basic info, as seen on screenshot 3.
The last screen shot is the details screen, and provides all the information about the trail that the app and users have added. Really helpful!
There is a pro version of this app that adds some features like being able to print and edit, as well as save, trails for your own good, but for the more casual hiker like me who just wants to find some new, local trails, the free version is perfect!