Long summer holidays -help please

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My dd's are 12 and 6 -and this causes horrible problems in the summer holidays as they inevitably won't play "nicely" together, appear to have very little in common and spend a lot of timing arguing and screaming at one another. Last year I spent a fortune on putting them into different summer holiday clubs and doing different activities to keep them occupied but I wasn't happy - I want to spend the summer hols with them doing family, fun stuff but it always seems to go horribly wrong when they are together. Well, it all starts again next week and I'm dreading it. Any ideas what I can do with the two of them without spending an arm and a leg but which will occupy them both happily? I do bike rides/picnics/etc and get lots of friends over but I'm at a loss really as to what to do for a very long 7 weeks! Thoughts?
Thanks

swimming? 12y/o can do what she wants whilst you play with 6 y/o.

an afternoon of baking?

a girly afternoon of movies and popcorn? shut all the curtains/darken the room so it like being at the cinema

check out your local leisure centre and churches as they usually have activities on.

you may want to spend all the holidays doing family things but you need time away from the kids and they need time away from you and each other. otherwise you will all argue. you probably need to look at doing something as a family a couple of time during the week, and a couple of times at an activity,a day in the house and then the normal weekend things of shopping etc