弟弟的暑假英语作文可以围绕他的个人经历和感受来写。以下是一个简单的例子，你可以根据弟弟的具体情况进行修改和补充：

Title: My Summer Vacation

During the summer vacation, I had a lot of fun and learned many new things. I started by going to the beach with my family. We built sandcastles, collected shells, and swam in the ocean. The water was warm and refreshing, and I loved the feeling of the waves hitting my feet.

One day, I also went on a trip to the mountains with my friends. We hiked through the forest, saw lots of wildlife, and took lots of pictures. It was a great adventure and I felt so alive.

In the evenings, I liked to stay at home and read books. I read a lot of interesting stories and learned new words. It was a good way to relax after a day of activities.

Another thing I did during the summer vacation was help my parents with the garden. We planted new flowers and vegetables, and watered them every day. It was hard work, but it was also rewarding to see the plants grow.

Overall, my summer vacation was a great time. I had fun, learned a lot, and made wonderful memories that I will cherish forever.

希望这个例子能帮助你为弟弟写一篇有趣的暑假英语作文!