I have twins, they are older now but when they were little I had a plan for every week (wet weather and dry weather versions)
So each day would have a morning and afternoon activity. One at home, one out of the house.
Dry weather out of the house activities: visiting different parks, walk in the woods to pick up leaves (to make a picture with later), walk by the river, visit to local NT gardens. Playing in the garden with the sandpit etc
Wet weather out of the house activities: visit to museum, visit to art galleries, visit to local castle, visit to the library, a trip into town to go to a cafe. A trip to the local garden centre and look at the fish. A trip to a friend's house or to see a relative.
Stay at home activities: painting, baking, building something, playing with cars or trains sets, messy play with jelly and squirty cream. Reading, singing and making music.
None of it was very expensive (didn't have much money at the time) but we always filled our days and had fun.