if body building isn't really a goal for you, i would look into some more functional workouts instead of breaking it up into body-parts like that. your entire body should be trained to work together. since you're looking at getting leaner, i would look at some CrossFit type workouts. the plateau effect is something that shouldn't happen. your body should grow just like the first two weeks in the gym, even 6 months down the road. you should see big gains every week. to do that your training has to be broad, general, and always changing. this can be a lot of fun when making up your workouts because you can get creative with the combinations of movements you put together. if you want some help on some workout ideas, just PM me.