Nice trip, were envious!
In order to do a turn in Davis, which is a much better place to wait on the Zephyr as Tricia said, you woulf have to pay for Coach tickets Round Trip from SAC to Davis on either the Starlight ( chances are you can stay in your room and occupy it upon boarding the Zephyr if none-one has it booked to/from SAC) or a Capitol Corridor Corridor train since AGR won't allow the connection in Davis, Martinez or Emeryville like in the past. If the Starlight is late to very late you can bail in SAC and cancel your Coach tickets.
If you do the SAC layover, Perkos, as was said, is fine for Breakfast. There is also a Starbucks close to the station if you like them!
In Galesburg there is a really good Mom and Pop place called "Baked"( Pizza and American) close to the Station on S.Seminary along with a Pizza Joint, Veggie/Vegan Place, a Deli etc.
The "Packing House" is across from the Station and is good, but is more upscale and pricey.
Check out Trip Advisor for reviews!