During the winter, Tenney Park has the best ice skating in the city. Huge pond and generally well kept ice. Skate rental is $6 for an hour. Very cool.
During the summer, it's a bit run-down and outdated. A cool running path and some basketball/soccer courts are in the middle of a moat-like pond. However, thanks to the pond, there are lots of mosquitoes.
The beach, pier, and locks are awesome to check out. Lots of fisherman, but generally quiet and a neat view of the city.