You have the right attitude. No point in pushing formal learning over the summer but let them guide their own interests through reading, visits to places of interest etc.
Do you have a local library? (I know many are sadly closed). Most librarys offer a summer reading challenge and these are usually promoted in local primary schools. If not, there are usually ones you can find online with different challenges to complete over the 6 weeks (discover a new author, write a book review, learn and perform a poem, read an autobiography, read a non-fiction book about a topic of interest, read in an unusual place, build your own reading den etc - you could even design your own set of challenges together).
There's also lists online like 101 things to do before you leave primary school (fly a kite, climb a tree, go camping, play pooh sticks etc, those sort of things) You could encourage him to make his way through one of those lists? Or again you could get him to make his own. A bucket list for summer 2024.
A journal is a nice idea to encourage but guided by whenever your child wishes to record, rather than something they feel they have to do each week.
Children who have access to books, paper and pens etc generally don't lose too many skills over the 6 week holiday. It's always noticeable who has not picked up a pen or a book for 6 whole weeks, but those who do generally settle back fine despite the lengthy break from formal schooling. The break does them good. It's not often that non-home schooled children get to guide their own learning. Six weeks is a long time to not do anything accademic though, so you'll usually find that children who are keen learners start to find educational ways to entertain themselves over the holidays, if given access to the right equipment (writing stories, making comics/magazines, designing their own board games, researching topics of interest etc. You can definitely facilitate this type of activity and encourage it without forcing it.