My two youngest granddaughters, ages 3 and 5, live up the street, and while they are both busy with their school and friends all week.. they come to spend Saturday with 'Grandma'. They love one on one time, and doing basically all the simple things. It's easy to entertain them.
We color together, water paint, play in the sandbox I have for them out back, read books, and they love card and board games that are age appropriate. In cooler weather, we bake cookies.. They get a kick out of looking for any fruit/veggies they can pick in the yard and take home. Ditto for flowers! I try to explain that the flowers are better in the ground, than plunked to only die. They have a hard time with that reasoning!
The only hard thing this year was when their Grandpa died early in March. Coming to the house at first was a little awkward for them. But things are okay again, and I think we've talked it through as much as you can talk to a 5 and 3 year old about such things. I let them know they are free to ask and say all that's on their mind. It's worked out.
Love having them over ... but dogs can get a little jealous at times.