Following and also posting in solidarity OP, we went on holiday last year and will go next year but I also have 3 young kids and am worried about the summer holidays too. We have an extra challenge in that we don't even have a garden 😬 we do have a park and large playing field just behind us though so will spend hours out there in the evening.
Ideas I have:
Get my neighbour to stop parking their extra car on my car port, and use the space as a garden sort of area. Get a mud kitchen or similar.
Lots of sensory play items bought ahead and hidden so I can bring things out strategically. Slime, sand, bubbles, playdough, fidget toys, etc. Mix of premade stuff and stuff to make together like moonsand or homemade slime.
Ikea near us is good for a day out. Creche for one hour while I grab a drink, cafe for lunch, let them maybe get a little thing like a small toy or some craft supplies, and then ours has a small park outside too. Very cheap day out!
We are lucky that we live near our city centre and also a lake with a huge park and facilities. As long as a take a picnic those can be great days out, will get a pop up sun shade and bring a book.
Board games the kids can play themselves with minimal supervision. We have pokemon labyrinth and will get some orchard games.
Plan all your resources, everything from treat meals to picnic lunches to spare cash for taking them out to the shops to all toys and craft supplies you have, and plan it on a calendar so it provides that structure and plan they are looking for and take the pressure off a bit. Even if one of those days is just "make a traybake together in the morning and take it out as part of a picnic with a loaf of bread to feed the ducks".
And just a reminder, it's okay for summer holidays to not be expensive days out and ling excursions. It's okay to spend the day making a bucket of bubble mix with a bunch of different bubble sticks, or set up a homemade cafe (my kids LOVE doing this, even if me and their dad are their only customers)