Wasn't really planning on working out today, but I was driving by one of my gyms and happened to be in workout clothes (it's the weekend), and I had 30 minutes free. I lifted hard yesterday, so figured I'd just do some interval work. I did 3 different ones; you could do any of the three, or all of them together.

One

Rowing: Set the rowing machine to display your 500m/average and total meters rowed. Row 200 meters at a 3:00/500m pace to warm up, and from then on go all-out every 100m. So, my rowing interval went:
-200 m at a 3:00/500m pace, 100 m at a 1:50 pace, 100 m at a 3:00 pace, 100 m at a 1:50 pace...and so on for about 15 minutes.

Two

Stair Treadmill: (BTW, I hate this thing, but it's good for you...) Set the stairmill workout for interval, then go as high as you can on the 'work' intervals and keep your rest intervals at like under 50 steps/min. I did 8 or 10 minutes here; it worked out to like 30 seconds alternating between 45 steps/min and 120/min.

Three

Ropes: You know those big thick ropes you can grab and swing up and down? Grab them. Find a way to time yourself and do 30 seconds max effort, 30 seconds rest alternating for 5 minutes. I usually do one set alternating arms as fast as possible and the next set slamming both ropes up and down at the same time as hard as possible. This is like sprinting for your shoulders. It sucks, but in an awesome way. If you're really going at it, lazy people stop what they're doing at the gym and watch you in awe.

Anyway, that whole interval workout took 30 min total, and there was enough variety and a short enough duration that it didn't feel like it was kicking my ass too bad, even though it was pretty hard. And you can scale this as much/little as you want.

Any other good ideas out there? Be creative!