If you haven't already, double-check your game settings to be sure the sims' autonomy and free will is set on high. Idle sims in an active household should be doing more than just taking care of their needs, even if it's just picking up a book, watching a tv, playing computer games, or something like that.
Beyond that, without mods, sims will most likely pick idle activities that suit their traits (athletic sims will work out or use gym equipment autonomously, outdoors loving sims will play on water slides or in the sprinkler, couch potatoes will watch TV, etc.) but they generally won't do much skilling up on their own. Objects that have a high fun rating will draw them in also.
