Are you going up 31 to 196 and picking up 131 at Grand Rapids? If so, you should find tons of stuff about the Blue Star and Red Arrow highways. If you have something specific in mind (burgers, fruit stands, pies, ice cream, etc), ask about that and I'm sure there will be tons of recommendations.
I take the 94-196-131 route north to my family's cottage in MI. I typically make my stops along the Blue Star highway but two places I have bookmarked off 131 north of Grand Rapids are Fricano's Pizza in Grand Rapids and Latitudes Roadhouse & Steelhead Tavern in Howard City. Fricano's is west coast Michigan pizza, and yes, it is its own distinct style of pizza. It's sort of a small chain now with a few locations in western MI. I have been to the original location in Grand Haven, so I can't vouch for the location in GR, but if it's close to as good as the one in Grand Haven, I'd say it's definitely worth a stop.

Fricano's pizza - it's a very thin crust stye
I have not been to Latitudes, but it looks like a good place grab some seafood and beer.
Fricano's Pizza
http://www.fricanospizza.com/5808 Alpine Ave. NW • Comstock Park, MI (just north of Grand Rapids off 131)
Latitudes Roadhouse & Steelhead Tavern
http://latitudes-steelheads.com/101 East Edgerton Street
Howard City, MI