If staying at home I'd buy a splash mat (dog one as stronger lol) they're circular and the outer sprays up and the middle fills up like a splash pool,
A pop up tent (small) for picnics and dens full of cushions, blankets etc.. Make a fairy garden/big hotel out of twigs etc.... Chalk the patio, pavement paint the sheds/walls with water and big brushes. Treasure hunts, gather foliage make pictures, bark rubbing, planting, watering plants,
Outside
Local parks.. Picnics take bikes, scooters,kites, foam gliders(cheaper and a big love for children)
Woodland(den building on a bigger scale) bug hunts, etc
Streams.. If local to a stream invest in a Magnet fishing, my GC love it and we spend hours fishing for treasure.
Wellies for splashing in streams and inevitably puddles too....
Indoors
Dens with chairs, tables and sheets
Painting cheap and cheerful hobby craft activities
Puppet on lolly sticks, sock puppets,
Junk modeling with all your recycling.
Gloop (flour and water) for mark making
Pasta necklace, painted.
Watch stick man, find and stick and make a stick man.
Go on a bear hunt (story) use the house as the river, cave, snow storm etc... (can be done outside too)
Make bird feeders,
So many ideas sorry lol