I've found that I have better practice sessions when there's some sort of outline or goal I need to work towards to. I'm sure thats the case with most others. So I usually make myself a few timed drills, which just helps me focus a bit better
Anywho, this topic is for sharing similar ideas around.
One that I like doing a lot is taking an object from my desk; can be anything really, and setting a 30 minute timer and drawing the object from every angle possible.
Or if you're feeling up to it, drawing 1 of every item on your desk. Works best if you have a messy desk like me.
Or if you're feeling particularly masochistic, you could try drawing each item on your desk for 30 mins each. Consider that a marathon event, or the spartan exercise of drawing.
Feel free to share anything else! It can anything other than line drawing too.