I would read some (good) books, particularly those about identity and sense of belonging, because you'll be in need of good related texts for English (plus it helps you develop your creative writing ability, which is also an important aspect of year 11 English). Some such books include the hunger Games Trilogy, The Fault in Our Stars (by John Green), Schindlers Ark (the film Schindlers List - also a great related text - was based on this book) and the poetry and letters of Wilfred Owen are fantastic. If you know the English books you are to read, then get a head start on those.
If you are doing maths, I recommend you buy a year 11 maths book (the Accelerated Maths Learning textbook range are excellent) and work on getting ahead for at least half an hour or 45 minutes a day. KhanAcademy has pretty good videos to help with maths (free online) that you might find useful. Also, the Crash Course (Science) and Sci Show YouTube channels can be quite helpful for keeping your mind on science, and they're only short videos.
The holiday period is far too long to be taking a break from studying at this point in your schooling, so make sure you 'keep your mind oiled' as someone once said. Apart from that, rest, have fun, stay healthy and get to school next year refreshed. Have a good holiday