I agree, you sound like a lovely Grandma. I have only one new baby GS so have lots to learn, but I agree with lots of the suggestions here and think back to mine and my children’s childhoods and what they enjoyed. This includes helping with jobs, cooking, baking, crafts, dressing up, reading stories, simple gardening like sewing cress or sunflower seeds, singing songs and nursery rhymes and going out for nature walks, pointing out trees, plants and insects. Have fun and enjoy your time with them and it will show, they will love you and remember you for it.
As someone has already said in another post, I don’t remember my GPs ‘playing’ with me, but there was always something interesting to do visiting their houses, particularly my MGM who was a dressmaker and good cook. I remember spending lots of time sorting through her button box!
Have fun at your tea party, sounds brilliant!