Go to Home Depot, buy a couple bags of sand. I wrap mine in duct tape to keep them from exploding if/when I drop them. Lifting and throwing bags of sand is a great workout because the sand it always shifting. It helps build stability and real-world strength muscles, not show-room muscles. Grab a duffel bag and throw in a couple sandbags. Put it on your shoulders and trudge up and down the stairs until you can't anymore.

Really easy to do at home and cheap. By the time your strength outgrows the sandbags, you should be motivated enough to invest in free weights or get to the gym. Or not, but you'll still have the sandbags and the muscles that result.