My sister-in-law turned me on to the TRX system (for lack of a better term) that is really quite remarkable. Using two straps with handles, anchored to a post, top of door, etc, you run through a series of exercises using your own body weight as resistance.
Although I've only been at it for a few weeks now, I can tell you that it works rather well. The thing this tool does is recruits many of the small stabilizing muscles that you just can't with other conventional systems. Your trunk and abs are constantly engaged, and you'll be sweating bullets in no time!
I made my own set of straps based on my sister-in-laws set, using old mt. climbing webbing I had lying around.
This is something that could easily fit into a purse, let alone suitcase, briefcase, etc. Great tool for someone on the go. You could use this in any motel room, gym, inside secured by a door, or outside tied to a tree.
Well worth a look.