We haven't done a ton of exploring of Highland park but love what we've experienced so far. Walking around the reservoir is easy and there are a variety of other trails and walking paths for both walking and jogging. There is also a pedestrian/bicycle lane on the main road loop. There's a number of pavilions and other gathering areas - and always seems to be something fun going on in the summer.
My favorite park of the park is the Super Playground. This old school wooden structure is wonderful for kids - and we always find at least one playmate my son's age. Although there are wood chips in the immediately area, it is important to note that it can get pretty muddy and wet. There's a water fountain and portapotty right outside the playground as well.
I would say that Frick and Schenley are two of the more beautiful and well-groomed outdoors spaces in Pittsburgh; however, Highland Park is a great option for walkers and (the Super Playground) and kids. Check it out!