Some indoor things i tried the first time round:
Brought her slide and outdoor toys inside for the afternoon, she really enjoyed the whole "oh they don't belong in here" and played for a couple of hours. Trick is to make sure they go back outside to keep the novelty of it .
Rotated toys, I'm still doing this and every couple of weeks will replace her toy with one out the attic and keep cycling round, this is keeping things fresh.
Disco bath...didn't matter that it wasn't bath time, actually think she found it more fun because of that. Got some glow sticks and had a cheapy disco light, stuck on some kids disco music on the iPad.
We did lots of baking/cooking but not always cakes, we made homemade burgers, bread, pizzas, frittata etc.
I've stocked up on arts and craft things, lots of pottery painting things, we have a few fairy pottery things from B&M and going to do them and then take them to the fairy trail walk by us on a not so rubbish day, she can then go back and visit her fairies. (tip...if there is black paint in the selection...just don't give it to them...most of our creations are pure goth, lol)
Colour matching, put coloured paper or fabric down and then go round the house looking for things that colour.
It's really difficult, I really struggle in winter anyways and i am dreading this year...but i am trying to be prepared and just get through it. I do find the whole "back in my day we didn't have soft plays" a bit nonsensical...I even had my mum say it to me. But you did have social interaction, you could go into someone's house and have a cuppa and let your kids play with others for a couple of hours, this is a different thing and certain areas (like mine) that can't happen. I'm a single parent so this whole thing is extremely isolating but just have to remind yourself it's temporary and we will get through it and ultimately my kid has LOVED having that quality time with me.