My turtles really only like two types of greens that I've given them so far, romaine lettuce and butterhead lettuce. The butterhead they went insane for, so, maybe your turtle might like it a lot too.
Mine seem to also enjoy brightly colored foods. Things that are red, orange, blue or a bright green they seem to want to eat more than the nasty gray color of reptomin pellets.
You could also try blueberries once or twice a week, since they are also very good for shell health. You can freeze them so they last longer. Fruit seems to, from what I understand, hold nutrients after being frozen better than vegetables. Just make sure you thaw out the blueberries under a heat bulb before you give them to your turtles. Aside from frozen food being hard to chew, first time I fed them blueberries was when they had just come home from the store, and they were very cold. It wasn't until they took on the temperature of the water, and got warmer, that my turtles started eating them more.
Spike - Egyptian mau mix, 8 years old
Phryne - Japanese bobtail, 9 months old
Hurricane - RES, 8 yo, 6 1/2 in. long
Typhoon - RES/Map hybrid, 8 yo, 7 in. long
Sadie - RES, 20 yo, 10 in. long
Sophie - Colombian red tail boa, 5 yo, 5 ft. long