music, xylophone, yacht, watch, Korea, Seoul.
Basically, words with multi-letter dipthongs are often hard to guess, or words with "rare" letters like q,z, x, etc.
Or, you could stir up a bit of classroom controversy with words like:
Takeshima, Bacon Tomato Sandwich (BTS), aeroplane, colour, favourite, Corea
Personally, I like to do variations of "hangman". My elementary students love the "Hang pikachu" version, especially since I often use a pikachu stuffy in class (the student who's holding the stuffy isis the one that gets to answer the question, a system I use because I can never remember their names, and because they like catching the stuffy).
They also really like the version where I draw a cage or pit filled with slavering, tentacled mosters / sharks / tigers / whatever you can draw, and a set of stairs leading into it (usually 4 or 5 steps. Then I draw a little stick figure at the top stair (often one that resembles a random student). Each wrong letter moves the figure down one stair. At the end, there's usually a small pile of bones scattered around the pit, and several students who are peeved at their mates for deliberately "killing" them...
