The "No Day-Dreaming" Workout

Without a doubt, the best weight workouts I get are with a training partner. The adrenaline flows, you push each through that last rep, and it's great having someone to do "negatives," "pauses" and other unorthodox techniques with.
When alone, the toughest part is keeping a flow to the workout. How many times have you done a set on the bench, and then sat around thinking about all of your other problems in the world? Or got into a conversation with a friend that turned a one-minute rest period into three or five?
To alleviate that down period, I like to super-set a strength-training set with a minute of jump rope, 30 hard seconds on the speed bag, or a set of hanging leg raises for abs. Not only does my concentration level stay higher, I know I'm getting just enough "rest" between each set for my muscles to breathe, but not fully recover.
To keep it more challenging, say to yourself: "If I tangle up on the jump rope or lose rhythm on the speed bag within that prescribed time, you must go back to your strength set that much sooner." Guaranteed, you will be more careful not to lose it!
One of my favorite workouts along this vein is the pull-up/chin-up drop down. I start with 10 pull-ups, and do a minute of jump rope in-between. Then nine, all the way down to one, the last five sets with a slow "negative" drop. Then you go into ten chin-ups (a little easier), and again, take it all the way down to one.
Oh, and if I miss on the jump rope, back to the "pulls" or chins" that much sooner! Finish it out with five minutes straight jump rope. Modify numbers on the pulls and chins, dependent on your current ability.
There's an unlimited number of variations of that basic workout. Figure out what your goals are, what you like to do, and what your capabilities are. Mix and match from there!