if I only have about 30 minutes a day to exercise (I know, I know, but work, family, etc... it all gets in the way and I do my best to sneak out for a little bit and get something done), should I just be doing a rotation of weights that whole time?  

When I'm at the gym, I never see anything like that.  The weight section is full of very large people lifting very heavy things for a very long period of time.  The cardio section is full of either fat people (trying to lose weight) or thin people (training because they're runners/cyclists/etc.), but not muscle people.

So, I know that you have to lift a lot and have to do nutrition stuff etc to actually become a big muscle guy, and so it's not like I have to worry about getting big, but I'm curious what I would even do for a short weights routine.  If I just have a half hour or so, am I better off sweating with cardio for that time, or rotating a few weights for that time.  I don't have examples of the latter around the gym because everyone there who does weights is a big dude who lifts for 3 hours.