可以用 "Her classes are extremely interesting." 表示她的课非常有趣。
"Interesting" 这个词是英语里常用的形容词，意思是有趣的、引人入胜的。
例句：
The movie was so interesting that I couldn't take my eyes off the screen.
这部电影非常有趣，我看不走。
I find this topic extremely interesting and I want to learn more about it.
我认为这个话题非常有趣，我想了解更多。