Crossword Puzzle books are great on long plane trips! "Specially the ones with all the answers in the back.
My difficulty comes from the clue mst of the time. Sometimes the clues can really be obtuse. Drives me mad when I cheat, look up the answer, and find a common word I've figured out easily many times before from less obscure clues. Grrrrr! On a visit to UK I looked at the London TIMES crossword. EGAD! In two words : UM POSSIBLE!