Books, I'd keep any books I enjoy reading, like many of the Dr. Suess ones. With little ones they want the same ones read over and over. Best to have a collection of books so they can choose. My 3 year old grandson was visiting and each time he visits he has to pull out some very old books, older than me possibly, part of Dr. Suess series, one about going to the moon and one about living under the sea. I have no idea why he likes those so much, the pictures I think, but that is one of the first things he does when he visits is to find those same 2 books and someone to read to him. Books I almost gave away as they weren't my favorites. Another favorite of his is a little snap together train set that fits in a small carrying case.
Christmas 2000 we had an enormous ice storm. Cooked the ham on the grill/smoker outside. We were lucky and had our power back on within 3 days, close to interstate and underground power lines, many were without power and phone for weeks. My husband was called into work right after we opened presents. He called me soon after and said to fill the tub and everything up with water, as the power company informed the city that those with power soon would have none either and they had no idea when they could get power back on.
My kids were 4, 7, 10, 13, 16, and 2 college kids home for the holidays. No power of any kind. My husband spent that whole week at work, keeping the cities generators running. No video games, no tv's, stereo.
Yet when they talk about it, it was one of their favorite Christmas's. We played board games huddled under blankets, ate sandwiches, heated soup on the camp stove. Peaceful in fact, adventure for the kids. No interruptions once the branches quit snapping off the trees. It was also amazing how much more time we had in a day, without our modern time saving conveniences.
Along with books I would add board games, card games, and construction sets. Things they can enjoy together. Games I'd find these in metal or wooden boxes, last longer. When we go camping we take some card games with us and they take up less room. A lifesaver when storms crop up while tent camping with kids.