I'm either hot or cold when it comes to fitness and strength training. When I can't make it to the gym, or on a run, I tend to not do anything.
I realize it's much better to be lukewarn in this case (lol).
So I invented a "workout for when I miss my workout". I made it just short enough so that I actually want to do it. Cause on the days when you miss the gym, or miss a run, it's hard enough just convincing yourself to do something. If you don't have an option for the days on which you miss the gym, or miss a run, and if you're like most people with careers unrelated to fitness, you're probably not gonna do anything that day, which ain't good. We should all be doing SOME physical activity involving exerting your ENTIRE body in a repetitively paced and controlled manner, EVERY DAY. So for me it's gonna be this:
(you'll probably want to invest in at least some dumbbells and a pullup bar to have at home)
At a MINIMUM you can do reps of 10 in everything except situps (50) and pushups (20-30). Then you can cycle through that rotation of sets about 2-5 times, depending on how much energy you have.
For a more intense "workout for when you miss your workout", just increase the reps and sets, or cycles. But keep it in a cycle! I tried to set the order of the cycle up so the set you're doing gives the muscle you just worked in the last set a rest, and vice versa.
You could use my cycle or not, just come up with an order that makes sense. Try and make EVERY element of your workout useful, using energy most efficiently! That way your "workout for when you miss your workout" is something you actually can see yourself doing on the days you otherwise can't get out to your real routine.