Any berries are good, and tomatoes, and squashes. Birds will go after the berries, too. I planted collards one year: while the turtles didn't appear to be interested in the plants themselves, they sure enjoyed the caterpillars and that fed on the leaves!
I've found that leaf piles and rotting logs are enjoyed because they provide cover for worms, pill bugs, slugs and snails, all of which are savored by turtles. Occasionally the logs grow mushrooms, which are also eaten. And of course the leaves also cool the soil, so turtles will nestle in on hot days.
As time passes and the enclosure here gets more overgrown, I think that the turtles find more goodies to eat on their own, they have more places to hide, and they seem to be utilizing more of their habitat.