I remember hearing one of the more experienced people on the forum saying no more than 2 small pieces of fruit a week. Timber doesn't like fruit so that's never been an issue with me, ha. I try to give Timber a variety of things. He likes some vegetables cooked, others raw. You have to experiment, and don't assume if your bird rejects something once it is a forever thing. You can offer Timber the same food three times in a week, once he will eat it the other two times he will pass. In addition to pellets, veggies, and a quality seed mix, they also sometimes enjoy boiled or scrambled egg. Timber is partial to meat, and I'm not sure how common that is. Meat is fine, just watch out for added salt or sauces that are unhealthy. They often enjoy a chicken wing bone, for instance. I just buy a pack of frozen wings and boil one for Timber occasionally. I strip off most of the meat and give him the bone. If he's in the mood, he'll strip every bit of marrow out. It's fascinating to watch.
Look at the "stickies" on opening page of the forum for nutrition and diet info. There is a lot there. As Kasperhm mentioned, sprouted seeds are good for them, and usually well received. Many of the forum members make "chop" for their birds (also birdy bread) and there are recipes in the diet and nutrition area I just mentioned. Unfortunately for me, Timber doesn't like either one. However, he is extremely picky and naturally thin (according to the avian vet).