For some sweet ideas.
Make a plain muffin/buttercake mix and grate in peeled zucchini and some orange juice to replace some of the milk. You can add orange zest too if the zucchini seems strong flavoured.
Chocolate muffins - add mashed pumpkin or sweet potato and reduce the liquid ingredients a little. Some people also like this with beetroot - but I'm not one of them!
The old fave carrot cake, but add double the carrot the recipe says.
Also, my DD went through a phase of rejecting veggies - despite loving them up to that point. I always still dished up some vegetables onto her plate at every meal. I never made a big deal about her eating them, just put them there and when she was ready she started to eat them again.