In no particular order...
Have brunch at Cafe Bonngout - the scrambled eggs are good.
Wander through the Innenstadt or Altstadt.
Check out the 12th century cloister in the grounds of the Muenster Cathedral.
Find that book in Bouvier.
Take a long walk along the Rhine towards the former Bundestag.
Have a beer at the Alter Zoll biergarten or enjoy the afternoon sun at a riverside biergarten on the Beuel side.
Check out the view of Bonn from the roof of the Kunst Museum.
Catch a film at the Brotfabrik.