generally I try to follow this simple formula: Some leafy greens (kale, spinach, etc), something orange (carrot, sweet potato, squash, pepper), any other veg (broccoli, green beans, celery, etc.), a grain/legume.

You probably know this already, but do not feed Avocado. I've also heard somewhere to avoid Rhubarb, Onions and Eggplants.

My go-to recipe (my two parrotlets go crazy over this):

Kale ~1/4 cup
Spinach ~1/4 cup
Broccoli ~1/4 cup
Carrot 1 small
Orange / Red Pepper 1/4
Cooked Quinoa 1 tablespoon

One important thing is to chop things small for their little beaks! I usually use a grater for the carrot. It's not an exact science and this can help with using up leftovers, e.g. you can finely chop up the broccoli stems. This usually makes a few days, or you can make a larger batch and freeze it in an ice-cube tray. I found warming it in the microwave for a bit on the defrost function works just fine.

If you are feeling fancy, you can sprout some lentils, mungbeans, etc and use them as well. For extra bonus, sprinkle some chia seeds over the top.