Keyhole is something I use for every very young learner's class. It's a simple envelope that will hold most flashcards and is great for eliciting the language as a review.
All you have to do is print it out, cut out the black keyhole and staple the 2 pages together. The students then get a small view of the flashcard and must try and tell you what it is. You can also turn it round and slide the flashcard out at the top, giving your students an even smaller sliver of an image to guess.