Retention of information seems to be a common issue during the summer break for students. I would recommend a reading list of related subjects to enhance your learning. Improve your vocabulary and math skills. Work on memory games to hone your skills. There are many choices on this site and listed by grade level.
I recommend you read a daily newspaper to start, then get yourself some good books, maybe a mix of fiction and non-fiction. I like to listen to recorded books when I'm in the car or doing routine things like the dishes or ironing. You don't state your age so I can't be more specific, but your local librarian might be able to help. To help you retain what you've read, I'd recommend you keep a journal or diary if you don't already, and take notes. I have a bunch of "logs" I keep on my laptop where I take notes on things I want to remember too. Good luck!
You can always check out education.com's worksheets collection. Many of them are very quick and easy to complete. Even just doing one worksheet every week will be a tremendous help in keeping you from getting rusty over summer vacation!