Which dinosaurs were plant eaters?

Much the same things that herbivorous animals eat today, with the exception of grass, which, while scientists now think it existed in dinosaur times, didn't become the extensive ground cover we know today until after they'd gone. The big sauropods browsed on coniferous trees like firs and pines, but during the Cretaceous deciduous tree appeared, so the later beasties could have munched on acorns, walnuts and the forerunners of Apples and Pears. Many species of flowering plants have toxins, so there would have been a lot of experimenting and adjusting to the new diet.
Herbivorous dinosaurs ate a wide variety of plants. Some of the larger Jurassic dinosaurs are known to have fed primarily on podocarp and araucaria forests. Podocarps are conifers.