这个暑假很充实可以用英文表达为 "This summer vacation was very fulfilpng."

"Fulfilpng" 是形容词，意思是满足的，令人满意的。它来源于动词 "fulfill"，意思是完成，满足。例如：

I feel very fulfilled after completing that project. (我在完成那个项目后感到很满足。)
The job was fulfilpng, but I decided to move on to a new challenge. (这份工作很有意义，但我决定去接受新的挑战。)

其他表示 "充实" 的英文词汇还有：

Rewarding (有价值的，有意义的)
Satisfying (令人满意的)
Enriching (丰富的，使人受益的)

例句：

That trip to Europe was very rewarding. (那次去欧洲的旅行很有价值。)
I find working with children to be very satisfying. (我发现和儿童一起工作很有意义。)
I hope this course will be enriching for all of you. (我希望这门课对大家都有益。)