Jamison Square Fountain
The park is right on the Streetcar line, adjacent to both pizza by the slice, a drugstore, and fancy restaurants that would not appreciate wet clothing. There's lots of comfortable areas for people watching, including benches and grassy nolls with young trees. Car parking is all metered, and there's a fair amount of bike parking as well.
The one potential problem is that this fountain, like the others favored by residents for a quick (or prolonged) spraydown, is very popular. On a hot day it can draw hundreds of users from around the city.