HELP! Entertaining grandkids

Depending on the ages, I'd pop them into the garden- if you have one - to make their own games. A skipping rope is quite useful and a ball [though it might go over the fence/wall!]. If it's fine fill a bowl of water and make paper boats or see what floats and what sinks. Get some sand and some sieves / pots. Get them to check out the garden for beetles, slugs etc and watch them. Find out what birds visit the garden. Draw pictures.
If it rains pop on raincoats and collect water and continue to sail boats.
Indoor play can be jigsaws, painting, plasticine, cutting out pictures for collage .........
Personally I think they should also make up their own games whilst the grandma sits on a chair and reads a book - and has a cup of tea!
As an only child I used to play by myself a lot. My parents bought me a little tent which was fun.
I wouldn't suggest doing everything at once. It doesn't hurt to be bored for a while

I think the ages would be helpful.

Mine love to help in the garden, planting, digging, watering, looking for mini beasts etc. They may like their own little patch or just be happy to help you, depending on ages, frequency of visits and size of the garden

It's for all ages of grandchildren!

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Mine just loved the large paddling pool, the tent and garden picnics. I'd fill a tray with rice for the toy diggers (doesn't matter if it spills, the birds will clear it up) and buy lots of chalks, white and coloured, so they could draw on all the paving (the rain washes it away). Cooking something simple, cupcakes, scones or pancakes is always fun.

Trips to various parks, lakes and forests - taking food for the birds and squirrels, the library 'story time', rides along the cycle path (using my collection of bikes, scooters and tricycles from the boot fair) were popular.

On rainy days, simply watching a film together, at home, with home made popcorn. Sometimes we'd go to city farms and local attractions, museums etc.

All fairly relaxed and low key, though, leaving the complicated and expensive stuff to the parents, with many days just spent here at home (apart from walking the dog). I believe that it benefits them to be bored sometimes, just do some drawing or read a book. Boredom cultivates creativity. We really don't need to frantically keep them entertained!

My tip would be to pay for them to attend a summer play scheme, they prefer to play with other children.

Germanshepherdsmum

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Yes, that's correct. I will of course contact anybody direct who responds here to check they're happy with that!

I bought some fishing nets and took them to a park,they loved it when they caught some.
And the amount of people who spoke to all of us was amazing.
One elderly chap shouted how lovely to see an old pastime being done.
We got the nets from a high street and used a jam jar to put them in.
Great fun.

Frisbee/boomerangs and balloons inflated with air and tied to a string are good options

Balloons on string again?
How about a treasure hunt (objects in nature), mini-Olympic games, Or any of the many special exhibitions and library activities in most towns. Storytelling at libraries.
Horrible Histories, Hendon for example.

A walk round IKEA with lunch in the cafe can take up a few hours. Buy one of their big rolls of drawing paper and paints or felt tips then at home get them to lie down on the paper, draw round them, then they can paint their full length self portrait. That’s a whole day done. Also - charity shops with a few £s each is good for all ages. Even teens enjoy this and I’ve taught some of them to sew and make their own outfits.