Adding fenders would look cool. Try and figure out your color scheme and buy cable housing in that color. Look for used alloy wheels. As stated this bike is never going to be a top line racer or touring bike. I would fix it up to be ride-able and enjoy it for what it is. If you are looking to get more out of it than it is, you would be better off putting money into a higher quality Schwinn or other brand bike.
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My bikes: 1970`s Roberts - 1981 Miyata 912 - 1980`s Ocshner (Chrome) - 1987 Schwinn Circuit - 1987 Schwinn Prologue - 1992 Schwinn Crosspoint - 1999 Schwinn Circuit - 2014 Cannondale Super Six EVO