@SkatingWithBears
Parks, forests etc are pretty rammed too...
There are plenty of places to go to that aren't, you just choose carefully. We have been out to the countryside and have barely seen anybody, certainly nowhere we have been going to has been rammed. We drove through a town in the Cotswolds on the way somewhere else and that was rammed but it's always rammed and you wouldn't choose to go there if you wanted a quiet place at any time let alone now.
If all else fails, offer a bored child some helpful and constructive domestic task.
This always works. Tidying their room is a suggestion which immediately results in them finding something to do round here 
It's harder as they get older as they aren't impressed with a picnic at the park and they end up wanting to play on screens all day, because that's what their mates are doing.
There is no reason why they can't enjoy a picnic in the park. You just mix it with other activities as well, my teenager likes to play on the console with his mates but it's not all that he enjoys doing. I just asked him what he'd think if it was all he did all summer, he wouldn't prefer it as it'd be boring.
I have a toddler and a baby. There is nowhere affordable to go when it is very hot or raining for days on end, it’s all barred to toddlers, and I can’t afford to take them to the zoo or a NT property every day.
You don't need to entertain the baby, a cuddle and a toy with interesting texture or time looking at pictures or flowers will do that, they lose interest very quickly and so you can entertain them easily - a bunch of keys, a saucepan, anything can be a fun toy.
As for the toddler, simple pleasures - a bucket of water in hot weather, do some gardening (plant a window box if you don't have a garden, make mud pies, show them how you make daisy chains (you can do it with dandelions for hands that can't do it with daisies), go on walks,
finger/hand/foot painting, make your own play dough, sack races (use an old pillowcase) - how fast can they get down the garden/lounge and back again. Nature treasure hunts - can they find 5 leaves with yellow on, 2 acorns etc. Lie on the grass and look at the sky - how many different animals can they see in the clouds.
Even with a car, driving 50 miles there and 50 back can make a dent in someone’s tight budget
We camp for weekends at a site less than 10 miles from home. It costs £10 for the site, we take a picnic with us for dinner on the first night, we don't have a cool box but full a bucket with cold water and put drinks in that (long life milk, bottle of water filled at home before we leave). We take the oven shelf and an old saucepan and make one pot meals over a fire pit sometimes but other (most) times we have cold meats and cheeses picked up from a local shop, some crusty bread rolls, bit of fruit. No cooking required.
We don't have the money to do what most of you do to entertain your children in a normal summer, we just use our imagination and have a good time each summer. Each summer it amazes me how much money people throw at entertaining their children when it's not necessary.