Upgrade and relocate the city's sewer treatment plant. Get it out of that prime spot between the bridges and make that space part of the continuous stretch of parkland along the riverfront. The benefits would be environmental (a modernized sewer system, and one less in danger of being flooded in our ever-more-common "100-year" floods), aesthetic, and recreational.
Really want to get serious about downtown redevelopment? Pick half a block in the downtown core (Fifth to Eighth, Maine to Broadway), buy out every building owner, and flatten. Then build a three-story,
free,
covered parking garage. Or if not free, permit merchants to validate parking. Can't be farther west than Third, because lazy Quincyans will not walk uphill. The north side of Maine between Sixth and Seventh looks like a good place to me, or Hampshire between Fifth and Sixth. Maybe the west half of the block on Maine between Seventh and Eighth.
Make sure that every street in the city – every street – has a sidewalk along at least one side. Make it easier for people to walk in their neighborhoods. Make it safer for kids to ride their bikes. And the heck with this business of a sidewalk that goes partway down a block, then stops, then starts again three houses down.
Convert to solar-powered and/or LED streetlights.