I’m all seriousness, the things that have worked well for us that hasn’t already been mentioned are:
Huntung for the perfect stick to make a stick man, then you can put goggly eyes on when you get home.
I gave them all a camera (old digital to 4 year old, kids camera to 1 year old and phone to 3 year old) and went on a photography walk in a wood. They loved that and bonus, we even have a few pics with me in them. When we got home I showed the older one how to edit the pics which he loved.
I’ve set up a play shop and given them real cans of food, pasta and empty shampoo bottles, a bag for life and real money, the two little ones play for hours with that filling and refilling - not all the same time but they do go back to it.
I made a water wall from half an old cot and drain pipes and jugs and funnels, they liked that and it was limited participation from me.
One total winner was this play foam, you just put an egg cup full hot water and fairy liquid into a blender and it foams up and is messy but clean mess. I put it on a tray each for them. That lasted 15 mins.
Cornflower isn’t nearly as bad as you think it will be and cleans up quite easily. They loved that for 10-15 mins when I gave them utensils.
Today I put mine in waterproofs and took them to an area that I know floods and let them go for it in the puddles. They loved that. I took spare clothes and a plastic bag and changed their clothes one by one before they went in the car. That one is messy though.
You can get giant bubble wands and they love those. That requires your input though but they will run round the garden trying to pop those for half an hour and they look magical.
I got some water colour paints from Poundland, not nearly as messy as real paint.
Our older one really enjoyed bark rubbing, you might be able to get both of them doing that depending on what your 2.5 year old is like. You can do that inside with leaves you e collected, coins etc.
We spent half an hour sitting outside the front of the back as opposed to the back. They actually really enjoyed the change of scene.
In nicer weather all mine enjoyed washing the car- again requires supervision.
This isn’t an activity, but when we can, sometimes my husband and I just take it in turns to have them for an hour and lie in a different room and look at our phones. That means over 4 hours, you would only need to think of stuff for 2.
Honestly, this is hardcore if you have toddlers. I feel like I’m cleaning and tidying all the time. Good luck, we can do it!