Buy some puddle jumpers/playsuits. Skin is waterproof!!!
Buy a small outdoor garden play pack with mini cones, bean bags, rings etc turn your garden into an obstacle course
Let dig and make mud pies in the garden, devote a corner for messy play
Finger painting, fruit/veg/leaves/flower stamping
Pasta and glue pictures
Make toys out of recycling Pinterest is great for this
Make salt dough and dry then paint
Make sun catchers and dream catchers
Nature trail treasure hunt
Geo caching there's free apps available
Bike rides, walks, parks
Splash in the sprinkler
Torchlight silhouettes pictures
Orchard boardgames- get these on ebay or charity shops
Make a picnic then choose a silly spot to have it
Play hairdressers or dress up
Make Shoe box dioramas
Paint stones and then hide them. See if you can find any
Pond dipping
Look for fossils or interesting stones
Popcorn garlands aren't just for Christmas
Paper chains
Paper lanterns and decorating
No bake baking ie. Rice crispy treats
Make instruments out of recycling and put on a show.
Make slime if you're brave enough....
The list goes on and on. Had the added benefit of going through lockdowns with a 4 then 5 year old. The above cost very little or a bit but if used more than once will be worth it. Charity shops and ebay are great for finding bits and pieces. Pintest is great as are websites like free things to do with the family in Surrey. Pack a lunch whenever possible and get the kids involved with that as well.