Kettlebells are compact, safe-ish, and pretty fun. I regularly work out with barbells now at the gym (roughly following Starting Strength), but before I started that, I spent a lot of time swinging a 35 lb kettlebell at home, which firmed up my back and shoulder muscles a lot. I recently added 20 lb and 60 lb kettlebells to the set, as 35 lb isn't a challenge to swing, but I can't yet consistently control it with one arm above my head.
+1 for kettlebells. They are particularly great since they are really economical (mostly since you do not need many). I've been using a 35lb one for my morning workouts. The coverage you can get with a kettlebell in a short amount of time just can't be beat.
