Mine eat; bell peppers, tomato (end slice with no seeds), sweet potato (might need to be cooked to soften slightly), apple, broccoli stalks (split and blanched), peas (lightly cooked and deshelled), melon rinds, lettuce (a good green one, not iceberg, blanched) . You can try carrot, lightly cooked, but none of my plecs touch it.
'Blanching' just means putting it in a jug or cup, pouring boiling water over it and leaving it to soften. The time it takes to make a cup of tea for yourself is about the right length of time to leave it in the hot water, then just drain it and let it cool.
Stab the veg with a stainless steel teaspoon to get it to sink