A reasonable sized flat-bar 12" to 18" can do a lot of jobs well but still ride in the bottom of a gym bag. You may want to slip it into a tube sock, one from each end, to disguise it and keep it from clanking when you put down the bag. Longer is better but even the short 12" bar can give you some serious mechanical advantage.

A flat bar is great for opening stuck doors or windows (as is common after an earthquake), for pounding through drywall, as a club, and as a wedge to hold a door closed if someone goes postal.

http://www.amazon.com/15-Utility-Wonder-Bar-PB/dp/B002GQ1OXS

A pair of underwear, socks, and a tee shirt, and ideally a travel-size deodorant, are almost as good as a shower. Doesn't take a disaster for those spares to come in handy.

Edited to add link.