Between Montreal and Quebec City on 40 on a Wednesday...
We can certainly pack a picnic, but we'd also go out of our way for a sweet lunch spot. I tend toward informal, but we eat just about anything & in a wide price range. Anything we shouldn't miss?
I'd suggest that you take the 138 instead. There are numerous cantines along the way that all will have something endearing if not delicious about them.
Some to keep an eye out for are (in no particular order) Chez Cocotte in Lanoraie, Casse-Croute Tournesol in Sainte-Anne-de-la-Pérade, La P'tite Bouffe in Champlain, Casse-croûte de la Pointe in Sainte-Marthe-du-Cap, Casse-Croute Archie in Yamachiche, Maxime Demers in Yamachiche, Chez Patrice in Berthierville (plan on spending too much money and about an hour to 90 minutes at the Gilles Villeneuve museum as well) Chez Micheline in Saint-Augustin-de-Desmaures, Casse-Croûte de Neuville in (strangely enough) Neuville, L'Express Du Fleuve in Neuville, Les Delices De La Marguerite in Neuville, Chez Carmen in Donnacona, Casse-croute du cap in Portneuf and Le Toit Rouge in Sainte-Anne-de-la-Pérade.
And then in Trois Rivieres you also might want to check out The Musée québécois de culture populaire and the Sanctuaire Notre-Dame-du-Cap.
Drive carefully and if you do take the 138 plan on a very enjoyable 5 to 6 hours traveling to Quebec City. The river views are awesome, comparing poutines and guedilles is fun (if not silly) and if you ever feel like you're going to be late the 40 is only minutes away.
And then as a shot across the bow, avoid Madrid 2.0 at all costs. The dinosaurs are long dead and extinct.
I like La Caillette in Maskinongé, they make fresh cheese curds on site + it's a casse-croute so time to enjoy a good poutine and buy some extra fresh curds