The Whanganui River is the main feature of the city. You can easily walk and cycle alongside the northwest side of the river from the Upokongaro Cycle Bridge (pretty much the northern edge of the town) to North Mole. This is on a combination of shared trail, road and footpath.
The section of the river that passes downtown also has several businesses on the river edge along Taupo Quay, including the visitor centre and cafes.
Much of the southeastern side is also accessible, especially around Kowhai Park. There are three road bridges with pedestrian access that cross the river. The Dublin Street bridge, several blocks up the river from downtown, takes you directly to Kowhai Park, and the Whanganui City Bridge, at the end of Victoria Ave, takes you directly to the Durie Hill elevator.
The river often has a rather muddy look as it flows over soft mudstones rather than hard rock, but it looks blue enough earlier and later in the day, generally the best times for a river walk.