Living nearby, this is my escape to nature. The GAP trail is easily accessible from Downtown through the Southside and on to the Waterfront and beyond! The trail is nicely paved and makes a great location for walking / jogging, biking, and roller blading.
There are limited areas where you must cross streets, so it makes for a good location for beginner bikers or roller bladers. Some areas do have slight inclines, so also be prepared for that bit of athletic training.
Not all parts are well lit, so at dusk / dark keep in mind safety as the trail is secluded (benefit during the daytime!). The trail also runs near the train tracks at many parts, so be prepared to see a train (or two) during your outdoor time.