Hi,
Our Family are putting together a trip for Jan next year, which includes four nights in Salerno. (We have never been)
However, happy to share my planning in case it helps.
For our 3 days/4 nights we plan the following
Day one - Day trip to Pompeii and Herculaneum
Day two - Day trip to Paestum (Greek ruins about 1 hour south of the city- They look amazing) -Stop at a Buffalo Cheese tour on the way back
Afternoon - Sightseeing in Salerno - seeing the castle, cathedral and a walk around (We also plan on seeing the Christmas lights)
Check out these links for what to do in Salerno, I think you could get a good days worth of things to do here.
(https://www.livesalerno.com/what-to-see-in-a-day-in-salerno). (https://theculturetrip.com/europe/italy/articles/top-things-to-do-in-salerno-italy/)
Day three - Trip to Amalfi Coast (You could also do Capri in Summer)
For food, there appear to be heaps of restaurants in the old town. I just venture around google maps and have found a few Pizza and Pasta places. Others might have better recommendations.
We are staying near the Ferry terminal, however, I hear the old town is worth considering.
Please let me know how your trip goes, and any hidden gems you find out. Enjoy your trip.