Hi from Wisconsin,
Can you be wild and crazy? Do an overnight? I know you have a place you paid for in Granada. But really. 3 hours in the train...for a handful of hours. Cordoba is very different than Granada. (By the way, make sure you get to The Basílica de San Juan de Dios while in Granada. The cathedral in Granada is grand, but this basilica is seriously over the top. There are enough relic bones to create a full working skeleton and have pieces left over. And enough gold and silver to make a Spanish conquistador proud. )
The Cordoba train station is close to the old town so taking the train will be easy. If you are using your phone for maps, you are OK. If you use the city produced tourist maps, keep in mind where North is on their map.
The mosque/cathedral is worth spending some time inside, especially near closing hour when it empties out. Walking the streets can take some time. The Alcazar and the Palace. But seeing the place as the sunsets and after from the lit Roman bridge...what do they say...priceless.
Over night lets you see the city cool down, tourists go away, and the lit buildings create a hush.
Good food too.
wayne iNWI