My kids loved to plan a kind of themed restaurant day, they'd make placemats and menus from paper and help or make the dinner. The fave was a burger restaurant. They still talk about it.
Lunch at a park at least once a week - we've taken cooked sausages in a hot thermos to have hot dogs on the go.
Find some woods with a stream, especially if it's one that's not well known.
Free museums.
Rock painting and making a rock snake for the garden or leaving them for others to find.
Washing the car - kids love that.
Water fight day.
Sending postcards from anywhere you go, especially if they're involved in choosing the card, buying the stamp, posting it too.
Make a water wall from guttering and an old pallet for the garden. Fun to make and hours of fun.
Make a marble run from loo rolls and a cardboard box or two.
Science experiments - mentos in coke, the one that looks like a lava lamp with oil and an alka seltzer and coloured water - all on Pinterest and more!
Collect sea glass, drift wood and shells if you go to the beach and use it to craft with, we've made picture frames before.
Make ice lollies, ice cream - so easy with cream, condensed milk and some cookies.
Make ice cream in a ziplock bag
Make cardboard bracelets with double sided tape attached and stick on things you find on a walk, leaves daisies etc. we've done it with crowns.
Pyjama day
Library and babycino day
Make cakes for grandma or neighbour
Grow something - my son is older than yours now but still absolutely loves to grow vegetables
Little ones love 'painting! Everything in the garden with water - paintbrushes and water is all that's needed
Teddy bears picnic
Campfire with marshmallows/s'mores/baked bananas/campfire cones
Hope these help x