Well, thanks to the construction along almost all of I90, I didn't get to Rockford quite as early as I'd hoped, so instead of looking for one of the wonderful recommendations made above, I backtracked a few blocks (once I found the venue where I'd be speaking) to a place I had seen as I drove down Main St. -- Chiquita Food Market and Taqueria. Bright, cheerful market with a terrific little taqueria inside. I ordered a taco and a burrito, somehow thinking that, since the taco was $1.99, servings would be pretty small. Ha. The burrito (I forgot to ask the price, and none are posted) was $5.99 and was massive. The carne asada taco would probably have been enough for dinner, and I brought most of the burrito home. But both were excellent. Chips brought to the table were fresh and crisp, and the red and green sauces were both good and looked homemade. Nice people. Good food. Not a place for an important celebration, but great if you're in a hurry or have a craving for Mexican food and happen to be in Rockford.
Also, the sign said that on weekends, they have birria.
Chiquita Food Market
1414 S Main St
Rockford, IL 61102
For even faster food, there was a guy out front with a cart selling elote and a few other "street food" items.
Also worth mentioning is the venue -- Tinker Swiss Cottage Museum. If you get to Rockford or are passing through and want to see a fascinating, splendidly preserved slice of American history and Victorian charm, this is a place to stop. Really wonderful place, with an all-wood interior and all original furnishings. It's on the site where Rockford was founded. Lots of local history.
http://www.tinkercottage.com/about-tscmg.html