Salmon Street Springs
SW Naito (Front) at Salmon in Tom McColl Waterfront Park
get there via trimet
find a bike route
If you mention Portland fountains you can play in, Salmon Street Springs comes instantly to mind. Located in Tom McColl Waterfront Park at the end of Salmon Street, you can see the fountain from Broadway, and in warm weather, it is always full of people.
There are three cycles of the fountain, called misters, bollards, and wedding cake. (Bollards are the stubby things along the river that you can tie your giant ship to, or in my case, run into while dodging traffic on the waterfront). Here's a better description from Haschel47:
In the first setting, a tiny spray covers the central area. This setting is popular with little kids, because they can run through the water without fear of getting hurt. In the second, three circles of water jets in the middle shoot straight up. The closer the jets are to the outside, the lower they go. This creates an image resembling a wedding cake. This arrangment is a bit more dangerous; the central jet shoots with the force of a fire hose. Finally, in the third setting, water jets around the perimeter of the fountain arc inwards, creating a huge downfall of water in the middle. Although it is fun to stand in the middle and be bombarded by all of jets, this is not recommended for little kids.