First, I turned it inside out and taped it up, so that the outside would be plain brown box instead of carseat pictures. Theoretically, I could cover it with white paper and/or paint. I wanted to make it look like a spaceship. I haven't done anything like that yet. I used a big book, a smaller box, and Nate's Bumbo to draw shapes on the box. I cut them out with a knife (this is really good for your kitchen knives (not really)). I thought about cutting them out completely, but then I remembered that Nate loves LOVES opening and closing things. So I left each one attached on one side.
The biggest one, I cut out as a door. The Bumbo shape opposite the door is low enough to the ground that it can also be a door. The next day, we introduced Nate to the play house/space ship/box with holes cut in it. He was a little less than impressed at first, but I think he likes it. He really likes it when I peek at him through the flaps. He didn't get it the first few times. Rather than looking through the box at me, he would crawl around the box to find me. He caught on eventually. Peek-a-boo remains one of his favorite games.
It would really look cute if I covered it in white butcher paper, then outlined the flaps and holes with colored tape. Then, I could draw stuff on the paper. I don't really think that would make a big difference to the kid, though.