You can get in an awesome strength workout using just your own bodyweight for resistance. For example:
* pushups (modified/wall/incline pushups if necessary)
Resistance bands are cheap, versatile and portable/storable - in short, an excellent piece of equipment. Double them up or stand on them to shorten them to increase the resistance.
Take a browse through the Exercise Demos at www.sparkpeople.com/resource/exercise_demo
for some ideas.