I wanted to thank everyone for the great advice. I think I'm going to go with some mix between the deck of cards and resistance bands.
Beyond that, this whole conversation has caused me to question my gym membership. For awhile now, I've thought that a large part of the appeal was psychological--the gym makes you feel like you're doing something. In my experience, however, the most serious work comes, not in the gym, but at the dinner table.
I like my gym a lot. But in some ways, it feels like part of the broader con. I'm a regular, but I'd be very interested in what percentage of people who regularly pay their gym fees use their gym, say, three times a week.