My main purpose for treats is a homecoming reward too, though I sprinkle around outside to prevent aggression. I think that mealworms, white millet, sunflower seeds, and scratch are about all the treats I give (not together!). Our scratch has red millet in it so I sometimes give them that when more convenient and not hot out. While our chickens get all sorts of veggies and fruits, I was leery of that with the guineas, as my hubby has a lot of garden space. We do also grow clover and turnips for them, and they love foraging for those greens in winter.Our main treats: Millet, dried meal worms, and sunflower seeds (they don't like those as much but eat some of them - they come in a millet mix from Costco and that's why I get them)
Millet and mealworms every day for "homecoming" - they come home for that and all is well so far.
Watermelon is expensive since I buy organic only with few exceptions. They eat the rind and the inside and I often steal the inside for myself. I would worry about the sugar in the watermelon and yeast overgrowth, possibly, if fed too much. Our Guineas get it when it is very hot sometimes, if I have any, and I give it to them out of the fridge.
In the winter, baby salad greens, kale, lettuce, etc are sought after treats! I think you have to be careful with kale, forgot exactly why, so those treats I really use in moderation.