I'm a secondary school teacher. To a new Year 7 student, we would recommend spending the summer holidays to relax and explore, which might involve reading or researching (live or online) topics that interest them. Real life experiences, such as visiting a museum or an exciting place (city, beach, mountain, river, etc.), in any country, are great stimuli, but we do realise that parents need to work (and relax) and cannot provide a programme of intellectually valuable pursuits. Mastering something new or persevering to get better at something, such as a sport, chess, musical instrument, a handicraft, cooking, etc. or starting to learn a language, is good fun too - and often there's less time for this sort of activity once term begins.
As ever, we would recommend regular reading for pleasure, which might be novels or short stories or nonfiction books about places, people, topics, etc. Books on tape are also good (e.g. for long journeys).
For those who are very weak at Maths, some remedial work might be recommended to ensure that skills are not lost over the holiday (previous posters have suggested Maths resources).
Lastly, for the transition to secondary school, we would encourage students to start reading, watching or listening to the news and starting to discuss and form opinions about the world around them. The website "The Day" is a good one for this: theday.co.uk/