Ideas for Bergamo

Take the funicolare up to the Citta Alta and wander around. The Colleoni chapel is magnificent. There's also a citadel with a WW2 memorial that is a good vantage point for taking photos of the upper city. There are several good restaurants to have lunch up there - but if you want something cheap and quick I highly recommend Polentone https://goo.gl/maps/U1YWrMV1dfPkCneM6

The polenta taragna with cinghiale ragu was amazing

Bergamo is great. Spend ALL of the available sightseeing time up in old Città Alta - upper Bergamo - that's perched above the modern city. It's compact and easily walked, and just strolling about taking in the architecture can be enough! Some ideas here:

https://www.visitbergamo.net/en/upper-bergamo/

As well, check out the basilica, Colleoni Chapel, and baptistry:
https://www.visitbergamo.net/en/object-details/23-santa-maria-maggiore-basilica/
https://www.visitbergamo.net/en/object-details/25-battistero-(baptistery)/
https://www.visitbergamo.net/en/object-details/18-colleoni-chapel/

Walk the Venetian Walls and take in the expansive views:
https://www.visitbergamo.net/en/object-details/2979-venetian-walls-and-unesco/

Take the S. Vigilio funicular even higher for even lofter views, the ruin of San Vigilio Castle and pleasant walkabout:
https://www.visitbergamo.net/en/object-details/3369-s--vigilio-funicular/
https://www.visitbergamo.net/en/object-details/20-s--vigilio-castle/

We had a fun (and very good) dinner here: if at all possible, eat outside under the greenery-festooned canopies:
https://www.ilcircolinocittaalta.it/en/

Use a translation tool for the menu...
https://www.ilcircolinocittaalta.it/proposte/la-nostra-cucina.html

...and to read about the history of the building, and important role the co-op plays in the welfare of the local community:
https://www.cooperativacittaalta.it
https://www.cooperativacittaalta.it/#aree

The funicular is half way up the hill. Take Bus 1 from the station to the top of Alta.

It might be easy to miss, but look for the former bishop's palace, now The Historical Archives of the Diocese of Bergamo. Just inside is a foyer that is decorated throughout with amazing frescoes from the 1200s. Like other ideas suggested, it is in the upper old town. The frescoes were only rediscovered under whitewash in the late 1930s, so they look great.

Thanks for all the feedback!