Lettuce doesn't have much nutritional content, and with birds it can cause diarrhoea. I feed my dragons more vegetables than fruit. There are three basic things you need to control in their diet: Oxalic acid, Phosphorus and Potassium. The first can cause problems with Thyroid function in high quantities, the latter two are more to do with calcium absorption.
I give mine a mix of: Pineapple, tomato, nectarine, peaches, pear, apple, banana, rock melon, bok choi, pak choi, silverbeet, grape, orange, apricot, blackberries, blueberries, raspberries, cucumber, carrot, broccoli, squash, butternut pumpkin, peas, mung / green / kidney beans, parsley, basil, mint as well as dandelion and clover flowers. If you feed lots of things in small quantities (silverbeet, broccoli and banana) then the oxalic acid, phosphorus and potassium problems should be avoided. Should have mentioned: avoid Rhubarb, as the high amount of oxalic acid it contains may prove fatal.
If you find that they are not interested in their veggies, try chopping it up really finely and dropping it on to a plate in front of them - if it looks like it's alive they want to eat it. Either that or put maggots in it