Buy a paddling pool. Find a shady spot, fill it with water and add lots of toys that pour and squirt and bubble. DC and their friends would play in this for hours.
Or fill an old baby bath/washing up bowl with cool water and toys and put it on low blocks so toddlers can play with it but are not in danger of falling in.
Older kids like nerf gun water games.
Create a scavenger hunt of things they need to find or spot outside (in a shady wood if it'shot like this, but otherwise, anywhere.) A feather, white pebble, fircone, acorn, specific leaf, fruit, herb flower (from their own garden) specific insects, animals, colours of car etc. Some items to bring back, some to sketch on paper etc. Anyone who completes the hunt gets an ice lolly or icepop. Don't make it too easy or too hard.
Pokemon Go if that still exists. It got couch potato DS2 skipping around outside one summer.
Making mocktails/smoothies/slushies/ice lollies
If you have a stream nearby: Mudslides, dam building, paddling.
Create some summer challenges that suit their personalities. if they are bookish, read 3-10 books (according to whether they are bookworms or not) and write reviews of them. Or write a book. Help them come up with a world, a few characters, a problem they must overcome, and leave them to it.
If they are sporty/physical
Learn to do X number of pressups or lengths of a swimming pool or cycle (somewhere safe) a certain distance, or learn a difficult dance routine/style. Let them build up slowly and draw charts to mark their progress.