The first way to play is to ask the child to run to another part of the house (my kids don't cover their eyes well without peeking, and they enjoy adding some physical exercise to our games), and then remove one or two of the objects:
The second way to play is to scramble the order of the objects and have the child put them back in the correct order. This is even more of a challenge than figuring out what is missing.
Now my kids (3 years old) were typically not successful at this game beyond 4 items until I taught them a little memory trick... They go through the objects in order and make up a little silly story about them. I think this is my creative children's favorite part. For example, in the photo above, the story is about a hen who lives in a house instead of a barn. One day she loses her key because she forgot it in the red wagon. She becomes so upset that she loses a feather, which the queen finds. The queen pounds the drum to tell everyone in the land that she found a magic feather and is off on an adventure. Then she gets on the train to explore new lands. It's amazing how well these silly stories help with memory - my kids can usually play this game with 8-10 items fairly easily.
This easy, no-prep game is really fun to play, and the best part is that the kids are building all sorts of new synapses in their brains that they can later use for other learning tasks...