A 3D model which you can rotate and practice drawing from any angle. (Some models are parametrized and their proportions can be randomized.)
As adults, we see all the mistakes in our work. And it demotivates us.
Tom Fox mentions it in one of his online courses, too: don’t fret about mistakes in drawings. They are just drawings. You’ll make another one. As I wrote, you should go for quantity over quality , and quality will follow.
Children have it easier. But we can be like that, too! See my article on the one bad habit we have to unlearn.
Have fun in your sketchbook, and don’t fret about the mistakes.