Sadly, I don't seem to have a green thumb, so I typically forego plants if I can get away with it
However, I notice my turtles seem to like watching the fish, Yaji from the basking dock and Yumi from the underwater log, and they're probably fantasizing about nibbling on them, if only they were bigger and the fish were slower.
I have Green Tiger Barbs and 2 Sterbai's and even though the Sterbai's are slightly slow and clumsy, they're pretty alert, so usually drift away from Yaji and Yumi
All that being said, I think perhaps getting certain tank mates can help as well
A walk around a safe outdoor area should be fine once in a while, but definitely do things that would enrich the turtle's life, like litefoot mentioned, hiding their food.