Walk from Pioneer Square, from any eastward street, to the riverside. It's nice to walk alone to the river. Try Westside Riverwalk.
I first reached the Salmon Street Spring. Have dinner at the pub/cafe/restaurant there. I saw a newlywed couple taking wedding-gown pictures by the spring around dinnertime.
There are horse-drawn carts available for hire from the Residence Inn area near the marina. The horses will take you from that posh hotel area, along the waterfront park to the spring area. Some people prefer to take a ride on the Portland Spirit from the dock right next to the spring.
A relaxing walk.