Just interaction with them singing, dancing with music on, get a little light machine that the lights "dance around the room", reading books, playing on a mat. Buy a shower curtain to throw down if no outdoor space and do messy hand painting, shaving foam, play doh any messy play with different textures, toys that feel different brushes, knobbly things, ribbons, clothes, silks
Take them with you while doing your "chores" or "jobs" talking them through what you are doing. What colours things are you are putting in the wash, counting things, what things do.
Going out for walks, (with a friend or family member ) feeding the ducks, walks in the woods and just sitting watching the world go by on a bench, they just take everything in, anything new for them to see or the repetition of the same things is all good.
I hadn't starting going to any classes then, so they are the things we did.