You can access the Tony Knowles Coastal Trail at the end of 5th Avenue, near the downtown hotels. From there, you can take an easy stroll out to Westchester Lagoon. You will almost certainly see lots of bird species, from eagles to red-necked grebes and shore birds. In season, you can also see silver (coho) and pink (humpy) salmon entering the lagoon through the spillway from Cook Inlet. You might also observe a beaver or muskrat.
For a longer walk, or if you have rented a bike from one of the stands near the downtown trail access, follow the Coastal Trail past Westchester out to Earthquake Park – but be alert for moose, which are often seen on that part of the trail, although only rarely by the lagoon itself except in winter. More than once, I have had to slam on the brakes of my bike to avoid plowing into an unsuspecting moose along the trail. Give them some space.
If you have kids or adults who haven't lost their fascination with the universe, look for the big sun statue downtown and keep track of the planet displays as you head to the trail and out past the Lagoon and Earthquake Park. The time it takes you to walk to each display is about how long it takes light from the sun to reach that planet.