Check out the Royal Canadian air force's fitness ladder. You can find a version of it here:
http://www.fourmilab.ch/hackdiet/www/hackdietf.html
Click on "
What, Me Exercise?" and select "
The fitness ladder" for an intro. You are supposed to spend 15 minutes each session, and if the rung you're on takes less time, go up a rung, and if it's taking longer than 15 minutes go down a rung. The site uses frames so while I've linked to the sections in this paragraph, you might want to start at the link above and navigate around rather than clicking the links above.
There are similar programs elsewhere, but this one is relatively easy, and doesn't require any equipment or significant space (ie, can be done in quarters on a cramped ship).