(よてい) A very everyday sort of plan – go to the movies, go to karaoke with friends.   $$$
(けいかく) Pretty close to 予定 in meaning, but perhaps more elaborate – you’d use 計画 to say “I have a plan to get rich!” or to say “I planned out our whole vacation to 3 countries.” . .  you wouldn’t use 予定 for that.
(あん) An idea you just had – or a proposal you’re making – to specifically solve a problem.
(きかく) A corporate plan. I.e. to build a high-rise building by 2011, or to release a new type of car.
(さく) Never used by itself.