It all depends what you like.
The usual suggestions are exploring the old town, visiting one of the museums in Cologne (would highly recommend the Roman Germanic Museum opposite of the Dom), visit one of the many churches in Cologne, taking the cable car across the river, and walk back towards the Rail bridge Hohenzoller Bruecke (via the Rheinpark), and then cross the river again via the rail bridge.
I would definately recommend to go to one of the brewery houses for lunch to try the local food and the local beer). In the Old Town I usually suggest the Malzmuehle or Paeffgen (Brauhaus in der Salzgasse). But if you just want a drink, you could also head to the Frueh brewery house opposite of the Roncalli Platz (aka the square next to the Dom and the museum).
These are just a few suggestion, and you wouldnt be able to fit everything into one day (unless you only spend an hour at the Dom and the museum)
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