For example: Running or walking stairs. Unless you live and work in a ranch house, this one rarely gets done and it is an ass-buster! Even a walk up and down stairs for 20 minutes will leave you sweating. I often times find it more challenging than lots of machines that are out there. So BAM! That's a permanent excuse-killer for not doing some great aerobic work!
Another example: I found a welded bar of iron at the theatre where I do most of my work and it is PERFECT for chin-ups. I love the challenge of chins....and understand that they can be daunting. IF YOU CAN'T DO A CHINUP, just find something to hang on that can support you weight! Hang as long as you can. When you get stronger (and you will!) start pulling....somewhere down the line, you WILL do a pullup!
Anyway, when I know I am going to be pressed for time on a day I want to work out, I plan to use that chin up bar. It's right there, and the moment I am done with a show, I hit the bar (hee hee) Even if I'm a little hungry I wait to eat till the chins are done.....and if you don't want chins......you can do push ups ANYWHERE! When I drive back to NY to visit my family, every time the car stops, I do 50 push ups. It wakes you better than any coffee, and in one trip I can bang out 300 push ups.....
On any day if you need a workout and can't possibly get to a gym or even to you Project Gym at your house, BE CREATIVE!!! There is always a way!